BIOLOGY 

LIBRARY 

6 


BY  THE  SAME  AUTHOR. 

Evolution  of  To-Day. — A  Summary  of  the  Theory  of  Evo- 
lution as  held  by  modern  scientists,  and  an  account  of  the 
progress  made  through  the  investigations  and  discussions 
of  a  quarter  of  a  century.  Octavo  .  .  .  $1  75 

CONTENTS  :  Introduction —  What  is  Evolution?  —  Are 
Species  Mutable  ? — Classification  of  the  Organic  World — 
Life  During  the  Geological  Ages — Embryology — Geo- 
graphical Distribution — Darwin's  Explanation  of  Evolu- 
tion— More  Recent  Attempts  to  Explain  Evolution — The 
Evolution  of  Man. 

*'  There  have  been  so  many  volumes  upon  evolution  that  an  ordi- 
nary reader  may  be  inclined  to  overlook  this  of  Professor  Conn.  We 
warn  him,  however,  that  in  so  doing  he  is  sure  to  miss  a  rare  contribution. 
It  is  just  the  thing  to  set  a  layman  right  and  is  thoroughly  judicial.  It 
sets  down  the  general  trend  of  thinkers  as  to  evolution  and  Darwinism, 
finding  limits  to  both  and  marking  their  usefulness  when  properly 
employed." — Hartford  Post. 

"  Dr.  Conn  evidently  favors  the  theory,  but  he  does  not  write  as  a 
partisan  or  to  carry  a  point,  but  simply  to  show  what  has  been  the  result 
of  the  fruitful  labors  of  the  last  twenty-five  years.  As  a  devout  theist,  he 
considers  evolution  simply  a  method  of  creation,  and  does  not  believe  that 
this  derogates  from  the  glory  of  the  Divine  Architect." — N.  Y,  Observer. 

G.  P.  PUTNAM'S  SONS,  NEW  YORK  AND  LONDON. 


THE    LIVING  WORLD 


WHENCE  IT  CAME  AND    WHITHER 
IT  IS  DRIFTING 


A     REVIEW     OF     THE    SPECULATIONS    CONCERNING    THE    ORIGIN    AND 
SIGNIFICANCE   OF   LIFE   AND   OF   THE   FACTS  KNOWN   IN    RE- 
GARD   TO    ITS   DEVELOPMENT,    WITH   SUGGESTIONS 
AS    TO    THE   DIRECTION    IN    WHICH    THE 
DEVELOPMENT   IS    NOW  TENDING 


BY 


H.  W.  CONN 


PROFESSOR  OF  BIOLOGY  IN  WESLEYAN  UNIVERSITY 
AUTHOR  OF  "EVOLUTION  OF  TO-DAY" 


G.  P.  PUTNAM'S    SONS 

NEW  YORK  LONDON 

27    WEST   TWENTY -THIRD    ST.       27    KING    WILLIAM    ST.,    STRAND 

(£l)e  ^nithcrbocker  IJpress 
1891 


BIOLOGY 

LIBRARY 

G 


COPYRIGHT,  1891 

BY 
H.  W.  CONN 


ttbe  ftnfcfeerbocfeer  press,  t\ew  iJorfc 

Electrotyped,  Printed,  and  Bound  by 
G.  P.  Putnam's  Sons 


PREFACE. 

THE  world  of  to-day  is  the  product  of  the  past 
and  the  foundation  of  the  future.  We  who  live  to- 
day can  only  judge  of  the  past  by  inference  from  the 
present,  and  our  only  knowledge  of  the  future  is  ob- 
tained by  observing  the  direction  in  which  the  world 
seems  to  be  trending  as  judged  from  its  history. 
Since  life  is  the  crowning  phenomenon  of  nature  its 
study  leads  us  closest  to  the  hidden  secrets  of  crea- 
tion. The  history  of  man  has  long  been  studied  ; 
the  history  of  nature  leading  to  man  has  only  just 
begun  to  claim  attention.  It  is  not  long  that  man 
has  seen  that  history  could  be  learned  from  other 
sources  than  written  records,  and  it  is  only  a  few 
decades  since  he  has  conceived  the  idea  of  reading 
the  history  of  life  from  nature  itself.  For  half  a 
century  now  have  scientists  been  trying  to  pierce 
the  mysteries  connected  with  the  life  of  the  past. 
The  history  has  not  yet  by  any  means  been  read, 
but  enough  has  been  revealed  to  make  a  sketch  of  it, 
as  it  appears  to  the  biologist  to-day,  not  a  profitless 
undertaking. 

Of  the  history  which  can  be  written  to-day  much 
is  uncertain,  much  is  no  more  than  pure  hypothesis 

iii 


IV  PREFACE. 

and  speculation  variously  attested  by  miscellaneous 
facts ;  but,  on  the  other  hand,  much  is  more  firmly 
established  than  any  recorded  history  handed  down 
to  us  in  manuscript.  Some  parts  of  the  history  of 
life  must  rank  among  our  most  certain  facts  of 
knowledge.  In  the  following  outline  of  this  history 
the  endeavor  will  be  made  to  separate  carefully  those 
parts  which  are  speculative  from  those  which  are 
based  on  a  more  firm  foundation,  and  to  give  to  each 
part  the  value  it  deserves. 

It  is  well  to  note  at  the  outset  that  the  facts  as 
collected  prove  to  us  that  the  past  history  has  been 
a  continuous  one,  and  one  in  which  the  facts  follow 
each  other  with  such  logical  consecutiveness  that  it 
may  be  regarded  as  a  logical  whole.  The  history  of 
life  has  not  been  one  of  isolated  facts,  but  a  continu- 
ous flow,  in  which  each  step  is  foreshadowed  by  the 
one  immediately  preceding,  and  in  its  turn  foretells 
the  next  one.  This  condition  of  things  has  led  to  a 
development  in  the  life  of  the  world  and  to  a  series  of 
facts  which  science  has  termed  evolution.  Whatever 
be  our  philosophical  understanding  of  this  term,  the 
facts,  so  far  as  life  is  concerned,  are  beyond  question. 
The  history  of  life  has  been  one  of  the  development 
of  forms  and  types  from  each  other,  or  rather  from 
common  centres.  This  law  we  call  organic  evolu- 
tion. The  facts  upon  which  organic  evolution  is 
based  are  beyond  controversy,  although  there  may 
be  still  some  dispute  as  to  the  interpretation  of  the 
facts.  No  one  whose  judgment  means  anything 
in  the  scientific  world  questions  that  the  history  of 
life  has  been  one  of  growth  and  development  of 


PREFA  CE.  V 

types  from  common  centres.  To  this  extent  the 
theory  of  evolution  may  be  considered  as  proved  be- 
yond dispute.  In  the  following  pages,  therefore,  the 
development  of  types  from  centres  will  be  taken  for 
granted.  This  is  the  most  easily  described  by  the 
terms  development  and  evolution,  and  the  history  of 
life  will  therefore  be  outlined  as  one  of  evolution, 
without  implying  by  the  use  of  this  word  anything 
as  to  the  manner  of  this  evolution  or  as  to  its  philo- 
sophical significance. 

H.  W.  CONN. 

MlDDLETOWN,  July,   189!. 


CONTENTS. 


CHAPTER  PAGE 

I. — INTRODUCTION — SOURCES  OF  BIOLOGICAL 

HISTORY  .......  i 

II. — THE  ORIGIN  OF  LIFE  16 

III. — THE  ORIGIN  OF  THE  ANIMAL  KINGDOM     .  59 

IV. — THE  RECORD  FROM  FOSSILS        ...  87 

V. — A  VIEW  IN  PERSPECTIVE    .         .         .         .113 

VI. — A  VIEW  IN  PERSPECTIVE  (CONTINUED)           .  141 

VII. — HISTORY  OF  PLANTS 167 

VIII. — THE  FUTURE  OF  THE  LIVING  WORLD         .  177 

REFERENCES 189 

INDEX .         .  193 


THE  LIVING 


CHAPTER   I. 


INTRODUCTION — SOURCES  OF  BIOLOGICAL  HISTORY. 

To  discover  the  history  of  life  and  to  predict  its 
future  is  perhaps  the  chief  problem  of  biological 
science.  This  is  the  basis  of  the  discussion  of  or- 
ganic evolution  ;  this  gives  meaning  to  the  schemes 
of  classification  of  animals  and  plants  ;  this  is  the  in- 
spiration of  investigation  in  embryology  and  geol- 
ogy, of  experiments  on  spontaneous  generation,  and 
is  indeed  the  object  of  biological  discussion  gener- 
ally. Biologists  are  in  every  way  trying  to  discover 
how  life  arose  and  how  it  developed  into  its  present 
forms.  Every  source  of  evidence  that  can  bear  on 
the  question  is  probed-^the  microscope,  the  chem- 
ist's retort,  and  the  geologist's  hammer,  each  lending 
its  assistance.  There  is  of  course  no  written  history 
on  the  subject,  no  recorded  sources  from  which  to 
draw  anything  except  a  few  of  the  most  recent  facts. 
The  evidence  from  which  the  history  is  to  be  drawn 
must  be  taken  wholly  from  such  accidental  records 
as  nature  presents  charily  to  our  inspection.  The 


2  THE  LIVING  WORLD. 

indefiniteness  and  complexity  of  this  sort  of  evi- 
dence make  the  problem  a  very  difficult  one,  and  it 
is  not  to  be  wondered  at  if  as  yet  the  complete 
history  cannot  be  told.  But  if  the  record  is  dis- 
jointed, there  is,  on  the  other  hand,  one  advantage 
which  history  drawn  from  such  a  source  presents. 
Recorded  history  is  frequently  intentionally  falsified, 
and  more  often  written  so  as  to  give  personal  im- 
pressions of  the  historian  by  incorrectly  stating  the 
facts,  and  thus  making  it  impossible  to  determine 
the  truths  of  which  detailed  record  is  written. 
There  is  no  such  falsifying  possible  to  the  record 
given  of  the  history  of  life  written  in  nature.  We 
must  believe  nature  is  true,  or  give  up  all  hope  of 
knowledge.  While,  then,  the  complexity  of  the 
record  makes  the  interpretation  difficult,  the  impos- 
sibility of  nature  making  a  false  record  gives  to  the 
conclusions  that  are  reached  a  certain  security,  of 
which  the  biologist  is  proud  and  which  is  the  justi- 
fication of  his  claim  that  he  deals  only  with  facts. 

There  are  several  distinct  sources  in  the  realm  of 
scientific  facts  from  which  the  student  endeavors  to 
read  the  history  of  life.  Recorded  history  of  any 
sort,  of  course,  tells  us  almost  nothing.  A  few  facts 
concerning  the  stability  of  recent  species  we  do 
succeed  in  learning  from  the  monumental  records 
of  Egypt,  and  a  few  facts  in  the  history  of  man  are 
written,  but  this  is  all. 

Evidence  from  Fossils. 

The  most  valuable  source  of  evidence  from  which 
we  can  trace  the  history  of  life  of  past  ages  is  that 


INTRODUCTION.  3 

of  fossil  remains.  From  the  beginning  of  the  deposi- 
tion of  the  stratified  rocks  it  has  been  constantly 
happening  that  animals  and  plants  which  have  died 
have  been  preserved  in  muds  and  sediments.  These 
sediments  subsequently  harden  into  rocks,  and  the 
buried  animals  become  fossils.  From  the  fossils 
thus  buried  we  can  get  many  glimpses  of  the  life  of 
early  times,  and  we  have  only  to  examine  the  fossils 
preserved  in  the  successive  layers  of  rocks  to  be  able 
to  formulate  more  or  less  of  a  detailed  history  of 
the  life  of  geological  ages. 

Now  this  source  of  history  has  some  decided 
advantages.  In  the  first  place,  when  dealing  with 
fossils  we  are  dealing  with  actual  animals,  and  not 
with  a  record  simply.  When  we  find  a  fossil  we 
know  something  of  the  size,  shape,  and  appearance 
of  the  actual  animal  or  plant  that  once  lived,  and 
thus  we  need  not  confine  ourselves  to  general  ideas 
of  type.  We  implicitly  trust  the  evidence  given  us 
by  fossils,  and  do  not  have  to  ask  if  some  modifying 
circumstances  have  deceived  us.  When  we  find 
a  fossil  oyster  it  is  impossible  to  question  that 
oysters  were  in  existence  at  the  time  of  the  deposi- 
tion of  the  rocks  in  which  the  fossil  was  found,  so 
that  the  history  drawn  from  this  source  is  positive, 
so  far  as  it  goes. 

But,  on  the  other  hand,  although  fossils  give  us 
abundant  details,  the  history  which  can  be  drawn 
from  them  is  sometimes  nothing  more  than  a  history 
of  details,  lacking  in  the  perspective  view  that  makes 
up  a  true  history.  In  the  first  place,  taking  all  of 
the  layers  of  stratified  rocks  together,  we  do  not  by 


4  THE  LIVING  WORLD. 

any  means  have  a  continuous  record  of  geological 
ages.  The  layers  of  rocks  are  the  positive  points 
of  history,  but  they  are  separated  by  blank  periods 
of  which  no  history  has  been  preserved,  and  of 
whose  duration  even  we  can  get  no  estimate,  ex- 
cept that  they  were  extremely  long.  These  periods 
must  forever  be  blank,  and  it  was  during  these 
periods  unfortunately  that  the  greatest  changes  in 
the  history  of  life  occurred.  Secondly,  fossils  can 
give  us  history  of  those  animals  alone  which  have 
had  a  hard  skeleton.  The  hard  parts  of  animals  may 
readily  be  preserved  as  fossils,  but  not  the  soft  parts. 
Consequently,  of  the  orders  of  animals  having  no 
skeletons,  fossils  can  tell  us  almost  nothing.  Un- 
fortunately, too,  all  of  the  early  forms  of  life  agree 
in  having  slight  hard  parts  or  none.  The  Protozoa, 
Coelenterata,  and  Vermes  have  left  fossil  records  in 
only  a  few  cases.  We  are  now  learning  further  that 
the  animals  which  formed  the  beginning  of  the  large 
groups  were  usually  without  skeletons.  The  early 
coelenterates,  mollusks,  and  even  vertebrates  proba- 
bly had  no  skeletons  hard  enough  for  preservation. 
Again,  it  is  evident  that  the  early  representatives  of 
any  type  were  few  in  number,  and  their  chance  of 
preservation  was,  therefore,  slight.  Hence  it  fol- 
lows that  fossils  can  only  in  exceptional  cases  tell 
us  anything  of  the  early  history  and  development 
of  groups.  Further,  since  the  skeletons  alone  are 
preserved,  fossils  can  tell  us  little  of  the  develop- 
ment of  internal  structure  of  animals,  and  this  is, 
after  all,  probably,  the  principal  feature  of  import- 
ance^.  for  in  most  cases  it  seems  to  have  been  changes 


IN  TR  OD  UC  TION.  5 

in  the  anatomy  of  soft  parts  which  have  inaugurated 
all  of  the  larger  departures  in  animal  history.  Again, 
the  rocks  in  which  fossils  have  been  deposited  have 
been  frequently  subjected  to  the  modifying  effects 
of  heat  and  pressure  (metamorphosis).  Sometimes 
this  metamorphosis  has  completely  obliterated  what- 
ever fossils  may  have  been  in  them  originally,  and 
very  commonly  it  has  so  obscured  their  structure  as 
to  render  it  impossible  to  determine  very  definitely 
what  they  originally  were.  It  is  readily  seen  that, 
the  older  the  rocks  the  more  liable  they  will  be  to 
such  metamorphoses,  and  hence  the  farther  back 
into  history  we  go,  the  less  definite  becomes  the 
record.  Of  the  most  recent  epochs,  quite  a  complete 
history  is  obtainable,  but  as  we  go  back  the  record 
becomes  less  and  less  sure,  and  finally  it  stops  alto- 
gether. Of  the  earliest  history  of  life  fossils  give  us 
absolutely  no  trace,  and  it  is  even  true  that  fossils 
give  us  no  satisfactory  record  of  the  early  history  of 
any  of  the  groups  of  animals. 

From  all  this  it  will  appear  that  although  fossil 
history  is  very  definite  where  it  is  obtainable,  it  will 
at  best  be  a  disjointed  history  abounding  in  details 
at  some  points  and  lacking  at  others.  Some  small 
steps  in  the  history  will  be  given  in  the  most  minute 
particulars,  because  of  the  abundance  of  animals 
with  skeletons  to  represent  them,  while  other  im- 
mense epochs  will  be  entirely  unrecorded  from  the 
lack  of  proper  conditions  to  produce  and  preserve 
fossils  representing  the  period.  Moreover,  from  the 
great  metamorphosis  of  the  rocks  of  the  older  periods, 
the  study  of  fossils  will  give  us  absolutely  no  record 


6  THE   LIVING  WORLD. 

of  the  early  history  of  life  and  nothing  of  its  origin. 
As  we  shall  soon  see,  the  earliest  fossil  history  repre- 
sents a  period  when  there  was  already  a  well-devel- 
oped fauna,  and  this  could  of  course  not  have  been 
the  beginning  of  life. 

Evidence  from  Embryology  and  Anatomy. 

Some  other  source  of  evidence,  then,  is  needed  to 
assist  the  record  of  fossils,  and  especially  to  aid  in 
giving  the  history  of  the  earliest  periods.  At  this 
point  we  find  a  second  important  source  of  his- 
torical evidence  in  the  study  of  embryology.  This 
we  may  compare  to  a  written  history,  since  the  his- 
tory of  the  race  is  written  in  the  development  of  the 
individual,  and  since,  like  written  history,  it  is  open 
to  certain  forms  of  false  statement.  Embryology 
gives  us  a  record  of  past  animals,  and  does  not,  like 
paleontology,  offer  the  animals  themselves  for  in- 
spection. 

The  foundation  of  the  value  of  embryology  as  a 
source  of  history  is  based  upon  the'  fact  that  the 
embryology  of  an  animal  repeats  in  outline  the  his- 
tory of  its  ancestors.  This  fact  is  sometimes  called 
the  first  biological  law.  It  was  announced  early  in 
the  century  by  Von  Bear  and  Agassiz,  and  has  sub- 
sequently been  investigated  and  confirmed  by  scores 
of  naturalists  until  .it  is  safe  to  say  that  there  is  not 
a  fact  in  biological  science  that  rests  upon  surer 
basis.  It  is  hardly  necessary  here  to  enter  into  a 
discussion  of  the  facts  upon  which  this  law  is  based. 
For  our  purpose,  it  is  sufficient  to  state  that  the  law 


INTRODUCTION.  / 

is  universally  accepted  in  the  biological  world,  and 
we  may  assume  with  confidence  that  it  will  be  a  faith- 
ful guide  in  the  study  of  the  history  of  life,  at  least 
in  so  far  as  animals  are  concerned. 

Embryology,  as  a  source  of  history,  offers  certain 
advantages  over  the  record  of  fossiliferous  rocks, 
and  at  the  same  time  it  is  open  to  many  difficulties 
of  its  own.  If  the  embryology  of  an  animal  repeats 
the  past  history  of  its  ancestors,  and  if  all  animals 
have  descended  from  various  common  points  of 
origin  in  the  past,  it  would  seem  that  we  should 
only  have  to  study  the  embryology  of  a  few  animals 
from  each  great  type  of  the  life  in  order  to  deter- 
mine accurately  the  history  of  life  in  the  past  ages. 
There  are  two  facts,  however,  which  render  such  a 
simple  proceeding  impossible. 

First :  The  embryological  history  of  an  animal 
lasts  a  few  days  or  a  few  weeks  ;  the  past  history  of 
life  has  taken  thousands  of  centuries.  It  is  plain 
that  the  embryological  history  of  animals  is  too  short, 
and  the  past  history  of  the  life  of  the  world  is  too 
long  to  make  it  possible  that  the  former  should 
retain  a  record  of  all  the  details  of  the  latter.  In 
the  epitome  of  the  history  which  is  presented  to 
us  by  the  development  of  an  animal,  we  find  only 
a  few  of  the  salient  points  in  the  history  preserved, 
which  we  may  suppose  represent  such  epochs  of 
the  past  as  have  been  of  the  greatest  importance, 
and  have  therefore  left  the  most  lasting  impres- 
sion. In  the  embryological  history  we  find  that 
details  are  left  entirely  unrecorded.  The  study  of 
an  embryonic  stage  can  give  no  idea  of  the  actual 


8  THE  LIVING  WORLD. 

appearance  of  the  ancestors  which  it  represents. 
Size,  habits,  shape,  are  entirely  unknown,  and  fre- 
quently we  cannot  tell  whether  the  early  animals 
possessed  a  skeleton  or  a  protective  armor,  and 
numerous  other  points  are  left  entirely  without 
even  a  suggestion.  Embryology  does  give  us  gen- 
eral points  of  the  fundamental  structure  of  the 
early  types,  does  give  us  clearly  an  outline  of 
the  changes  through  which  animals  passed  in  their 
early  history,  does  give  us  an  epitome  of  the  early 
development  and  growth  of  living  things.  Such  a 
history  is  much  like  a  history  of  mankind  which 
should  give  an  outline  of  the  development  of  social 
relations  and  political  institutions,  which  should  tell 
that  the  family  relation  was  superseded  by  that  of 
the  tribe,  this  by  the  absolute  monarchy  and  the 
constitutional  monarchy,  and  finally  by  the  govern- 
ment by  the  people.  Such  a  history,  dwelling  more 
or  less  at  length  upon  these  various  forms  of  govern- 
ment and  showing  their  relations  as  well  as  their 
modes  of  origin  from  each  other,  would  be  much 
like  the  history  which  the  study  of  embryology  gives 
us  of  the  early  life.  A  history  giving  no  names,  no 
ideas  of  nations,  we  should  regard  as  a  poor  history 
of  nations  and  people,  but  it  might  be  a  fair  history 
of  mankind  in  general.  So  embryology  deals  in 
types  and  their  relations  and  origins,  but  tells  very 
little  of  actual  animals.  In  dealing  with  a  past 
ancestral  stage  thus  represented,  we  do  not  have  the 
same  sort  of  satisfaction  as  in  handling  a  fossil.  We 
do  not  know  just  what  sort  of  an  animal  was  really 
represented  by  the  stage  in  question,  but  we  do 


INTRODUCTION.  9 

have  the  satisfaction  of  feeling  that  we  are  dealing 
with  a  type  which  was  of  importance  in  the  history 
of  animals,  and  therefore  of  more  far-reaching  sig- 
nificance than  any  fossil  or  score  of  fossils  that  we 
could  pick  out  of  the  rocks.  The  relations  expressed 
by  corresponding  stages  of  different  animals  are  very 
suggestive,  and  a  short  study  of  embryology  will 
tell  more  of  the  true  history  of  animals  than  the 
collection  of  thousands  of  fossils. 

Second  :  The  second  fact  that  renders  the  study  of 
history  from  this  source  a  difficult  matter  is  the  very 
common  falsification  of  the  true  record.  It  is  un- 
doubtedly the  law  that  animals  tend  to  repeat  past 
history  in  their  embryology,  but  it  is  also  true  that  not 
infrequently  various  modifying  circumstances  occur 
to  prevent  the  history  being  correctly  recorded. 
The  true  record  of  past  history  is  thus  commonly 
obscured  by  numerous  departures  caused  by  the 
conditions  surrounding  the  embryo,  and  this  of 
course  renders  the  reading  of  the  history  a  difficult 
matter.  The  embryologist  has,  however,  methods 
of  correcting  these  errors  in  large  measure,  and  of 
reaching  trustworthy  conclusions. 

In  spite  of  these  two  objections,  embryology  is  of 
the  very  greatest  assistance  in  enabling  us  to  read 
the  past  history  of  animals,  and  to  a  less  extent  that 
of  plants.  Especially  is  this  true  of  the  early  stages 
of  the  history.  As  we  have  above  seen,  the  fossil 
records  of  animals  become  less  and  less  satisfactory 
as  we  go  farther  back  in  history,  and  they  finally 
cease  altogether,  long  before  we  come  to  anything 
like  a  union  of  the  converging  types  into  a  common 


10  THE  LIVING  WORLD. 

starting-point.  It  happens,  however,  that  it  is  of 
just  these  early  periods  that  embryology  gives  us  the 
clearest  account.  For  various  practical  reasons, 
embryologists  have  largely  confined  themselves  to 
the  study  of  the  early  stages  of  the  development. 
These  stages  teach  of  the  relations  of  types  to  each 
other,  and  of  the  separation  of  the  types  from  a 
common  starting-point.  In  other  words,  they  give 
us  an  idea  of  just  that  part  of  the  history  of  life  that 
we  fail  and  shall  forever  fail  to  get  from  the  study  of 
fossils. 

Closely  allied  to  the  study  of  embryology  is  that 
of  anatomy.  It  has  been  determined  that  the  em- 
bryological  history  of  the  higher  animals  of  a  class 
passes  through  stages  which  are  represented  by  the 
adults  of  lower  animals  of  the  same  class.  This  of 
course  renders  the  study  of  lower  types  a  valuable 
aid  in  tracing  the  history  of  the  higher  ones.  It 
is  also  recognized  to-day  that  relations  between 
animals  represent  community  of  descent.  When 
we  find  two  animals  closely  related  anatomically  we 
interpret  the  fact  as  indicating  a  recent  common 
point  of  origin.  Anatomical  relationship  thus  rep- 
resents blood  relationship,  and  if  we  have  the  former 
we  can  interpret  the  latter.  It  will,  of  course,  follow 
that  the  study  of  anatomical  relations  will  teach  us 
history. 

The  study  of  anatomy  and  embryology  cannot  be 
isolated  from  each  other.  Together  they  give  us  an 
idea  of  the  relations  of  types  to  each  other,  and 
enable  us  to  draw  up  a  picture  of  early  development 
of  life. 


INTRODUCTION.  II 

Miscellaneous  Evidence. 

But  even  when  we  carry  the  history  back  to  the 
limits  of  embryological  record,  we  fail  to  reach  the 
beginning.  Embryology  starts  with  the  living  cell, 
and  traces  the  growth  of  the  organic  world  from  this 
point.  Can  we  learn  anything  of  the  origin  of  this 
cell  ?  In  thus  going  back  to  the  very  beginnings  of 
the  history  of  life,  we  leave  fossils  and  embryos  far 
behind,  and  have  to  look  for  our  evidence  elsewhere. 
There  is  no  history  yet  discovered  of  the  earliest 
stages  of  the  living  world.  There  is  nothing  known 
in  nature  which  can  tell  when  life  first  appeared 
in  the  world,  or  how.  Upon  this  subject  we  can 
make  only  vague  conjectures,  instigated  by  various 
factors  of  nature.  That  in  some  way  living  things 
arose  from  that  which  was  not  living  is  certain,  for  at 
one  time  the  world  was  unfitted  from  its  molten  con- 
dition for  the  existence  of  life.  But  whether  life 
first  came  in  accordance  with  the  laws  of  nature 
which  are  still  in  operation,  or  in  accordance  with 
supernatural  laws,  is  still  a.  question  in  dispute,  and 
perhaps  may  always  remain  so.  All  the  evidence 
which  can  be  brought  to  bear  upon  the  subject  is 
indirect,  and  can  be  comprised  under  three  heads. 

i.  Experiments  upon  spontaneous  generation,  t.e.9 
experiments  to  determine  whether  life  can  to-day 
arise  from  the  non-living.  These  experiments  have 
been  performed  with  a  great  deal  of  care  and  perse- 
verance. They  are,  however,  entirely  incapable  of 
touching  the  real  question.  A  positive  or  negative 
conclusion  would  have  little  affected  the  great  ques- 
tion under  discussion.  At  the  present  time  they 


12  THE   LIVING  WORLD. 

have  been  decided  in  the  negative,  but  this  only 
shows  that  under  certain  conditions  (which  con- 
ditions were  doubtless  not  those  of  the  times  when 
life  first  appeared)  life  cannot  arise  spontaneously. 
They  tell  nothing  as  to  indefinite  unknown  condi- 
tions of  the  past.  If  they  had  been  decided  in  the 
affirmative,  they  could  show  only  that  life  could 
arise  under  certain  conditions,  which  conditions 
again  were  unquestionably  not  those  of  early  times. 
The  experiments  have  all  started  with  a  nutrient 
solution,  which  contained  already  products  which 
were  the  result  of  life,  z.r.,  meat  solutions,  etc.,  arid 
such  a  condition  would  of  course  have  been  impos- 
sible at  the  time  when  life  began.  At  best,  then,  the 
experiments  which  have  been  performed  upon  spon- 
taneous generations  could  serve  only  to  make  us  a 
little  better  acquainted  with  life  and  the  conditions 
under  which  it  now  acts,  without  helping  us  a  step 
toward  answering  the  question  of  its  primitive  origin. 
It  cannot  be  denied,  however,  that  if  this  subject 
had  been  or  ever  shall  be  decided  in  the  affirmative, 
it  would  render  the  origin  of  life  by  natural  laws 
more  probable,  since  it  would  show  that  living  things 
could  come  from  the  non-living,  and  this  would  be 
one  step  toward  the  solution. 

2.  Study  of  organic  and  physiological  chemistry. 
It  may  seem  somewhat  strange  to  count  chemistry 
as  a  source  of  biological  history,  and  of  course  the 
evidence  it  gives  is  only  indirect.  Chemistry  has 
been  for  some  years  now  teaching  us  of  the  close 
connection  between  chemical  and  biological  laws. 
It  has  shown  that  many  organic  bodies  can  be  pro- 


IN  TR  OD  UC  TION.  1 3 

duced  by  purely  chemical  processes.  It  has  shown 
that  many  of  the  vital  processes  of  living  things  are 
simply  chemical  in  their  nature,  and  that  some  of 
them  may  be  imitated  by  lifeless  material  in  the 
laboratory.  It  has  shown  that  chemical  compounds 
of  great  complexity  have  complex  properties,  and 
that  protoplasm  is  the  most  complex  substance  in 
existence.  In  all  of  these  ways  has  the  close  union 
of  chemical  and  biological  laws  been  made  evident ; 
and  chemistry  has  thus  prepared  the  way  for  the 
conception  of  a  still  closer  union,  and  for  the  sup- 
position that  life  originally,  as  now,  was  a  mere 
application  of  natural  chemical  laws  to  complex 
i  conditions,  and  thus  arose  by  natural  and  not  super- 
natural law. 

3.  The  study  of  the  low  forms  of  life.  This  is  of 
value  in  showing  the  simplest  conditions  of  life,  and 
therefore  bringing  us  nearer  to  the  condition  of  the 
first  life.  The  simplicity  both  of  structure  and 
function  of  some  of  the  lowest  forms  of  life  seems 
to  bring  us  very  close  to  the  inorganic  world.  The 
step  from  the  amoeba  to  some  inert  chemical  com- 
pound is  certainly  less  than  from  man  to  the  same 
compound.  From  the  study  of  these  simple  forms, 
we  can  easily  conceive  of  still  simpler  masses  of  proto- 
plasm with  even  less  organization  than  the  amoeba 
and  proteomixa.  At  the  same  time  the  complexity  of 
compounds  manufactured  by  the  chemist  seems  to 
be  approaching  the  somewhat  greater  complexity  of 
these  simplest  forms  of  life.  It  is  plain  enough  that 
the  simpler  the  condition  to  which  life  can  be 

reduced  and  the  smaller  the  gap  between  the  sim- 

?li  -^^ 


OF 

'UNIVERSITY)) 


14  THE  LIVING  WORLD. 

plest  living  thing  and  the  most  complex  compound 
not  alive,  the  less  will  be  the  difficulty  in  believing 
in  the  natural  origin  of  life.  If  we  could  find  a 
substance  that  was  simply  living  matter  with  no 
definite  characteristics  of  any  specific  nature,  this 
would  be  the  starting-point.  From  a  priori  grounds 
we  might  expect  that  simple  living  matter  would 
appear  before  any  definitely  formed  species.  This 
fact  is  the  basis  of  the  interest  connected  with  the 
study  and  speculation  concerning  protoplasm,  the 
supposed  common  factor  of  living  things.  In  the 
simplest  forms  of  life  we  get  down  almost  to  pure 
and  simple  protoplasm,  and  by  taking  from  these 
forms  all  of  the  common  factors,  we  may  suppose 
that  we  obtain  the  characters  of  simple  protoplasm 
itself,  and  thus  presumably  the  first  form  of  life. 

There  is  another  series  of  facts  which  can  hardly 
be  called  evidence,  but  which  does  at  the  same  time 
have  a  great  influence  in  our  interpretations  of  the 
past.  The  long-continued  study  of  nature  has  led 
to  the  formulation  of  the  law  of  continuity.  Ac- 
cording to  this  law,  the  processes  of  nature  have 
been  those  of  continual  slow  change,  such  that  the 
history  of  any  minute  is  explained  by  the  conditions 
of  the  preceding  minute.  The  law  admits  of  no 
great  breaks  in  the  history  of  the  processes  in  nature, 
but  assumes  that  where  such  breaks  seem  to  exist, 
the  break  is  only  in  our  knowledge,  and  not  in  the 
nature  itself.  It  is  impossible  that  the  acceptance 
of  this  law  should  fail  to  have  great  influence  in  the 
interpretation  of  the  life  history,  for  by  means  of  it 
a  constant  development  is  necessarily  substituted 


INTRODUCTION.  15 

for  the  series  of  epochs  which  the  actual  facts  seem 
sometimes  to  indicate.  Especially  is  this  true  of 
the  conclusions  as  to  the  origin  of  life,  for  here,  as 
we  have  seen,  direct  evidence  is  wanting,  and  the 
belief  in  the  law  of  continuity  forms  the  foundation 
of  all  that  is  to  be  said  on  the  subject. 

Such  are  the  sources  of  the  evidence  from  which 
our  history  of  living  nature  must  be  drawn.  Of  its 
beginnings  we  know  nothing  beyond  such  inferences 
as  may  be  drawn  from  experiments  on  spontaneous 
generation,  organic  chemistry,  and  the  study  of  the 
lowest  forms  of  life,  together  with  the  general  teach- 
ing of  the  law  of  continuity.  Once  established, 
however,  we  can  trace  in  outline  at  least  the  early 
history  of  animals  and  plants  through  the  ages  by 
means  of  the  record  we  have  of  such  history  in  their 
embryology  and  their  anatomical  relations.  Later 
the  stratified  rocks  begin  to  preserve  for  us  here  and 
there  a  scattered  page  of  history  ;  and  as  we  come 
to  later  ages,  the  leaves  thus  preserved  become  more 
and  more  perfect  until  in  the  latest  times  a  fairly 
complete  history  may  be  read  from  them.  Perhaps 
by  the  study  of  the  course  of  the  past  we  may  then 
be  able  to  hazard  a  prophecy  as  to  the  course  of  the 
life  of  the  world  in  the  future. 


CHAPTER   II.* 

THE    ORIGIN   OF   LIFE. 
What  is  Life  ? 

IN  taking  up  the  study  of  the  history  of  life,  we 
must  first  ask  the  question:  what  is  life?  This 
question  is  asked  not  in  expectation  that  any  satis- 
factory answer  is  possible,  but  in  order  that  we  may 
get  as  clearly  as  possible  before  our  minds  the  chief 
facts  in  modern  thought  concerning  the  subject.  It 
is  clearly  true  that  our  scientists  have  by  their  specu- 
lations and  experiments  so  completely  changed  our 
ideas  of  life  that  it  sometimes  seems  as  if  we  could 
almost  grasp  the  real  essence  of  the  matter.  The 
solution  of  the  life  question  is  said  to  be  close  at 
hand.  Tt  is  well  for  us  at  the  outset,  then,  to  review 
the  question,  to  see  where  we  stand  to-day  in  our 
knowledge  of  life,  and  to  notice  what  pointy  have 
been  settled  and  what  points  still  baffle  comprehen- 
sion. For,  thus  far,  this  life  essence  has  been  an 
ignis  fatuus  ;  and  although  many  preliminary  ques- 
tions have  been  solved  in  pursuit  of  it,  their  solutions 

*  The  substance  of  this  chapter  was  originally  published  in  the 
New  Princeton  Review. 

16 


THE    ORIGIN   OF  LIFE.  17 

only  serve  to  show  us  that  there  is  something  else 
beyond,  which  is  not  comprehended.  Our  first  task 
is  then  to  find  out  exactly  what  is  the  question  now 
at  issue  ;  for  it  is  very  different  from  what  it  used  to 
be.  We  shall  find  that  much  is  now  universally  con- 
ceded which  was  at  one  time  strenuously  disputed. 
There  are  three  essential  properties  possessed  by 
living  things  which  must  be  included  in  any  at- 
tempted explanation  of  life :  a.  Their  constant 
activity,  b.  Their  power  of  growth,  c.  Their  power 
of  reproducing  themselves.  These  being  the  essen- 
tial properties  of  life,  their  satisfactory  explanation 
will  bring  us  far  toward  the  understanding  of  life 
itself. 

Relations  of  Organic  Activities  to  Physical  Energy. 

First  we  may  say  that  the  activities  of  organisms 
are  no  longer  looked  upon  as  manifestations  of  a 
distinct  "  vital  force  "  unrelated  to  other  forces.  It 
will  hardly  be  denied  by  any  one  to-day  that  all  of 
the  energy  exhibited  by  organisms  in  their  various 
activities  is  a  part  of  the  store  of  energy  of  the  uni- 
verse, and  that  all  of  the  forces  exhibited  by  animals 
are  correlated  with  physical  forces  in  general.  It 
has  been  conclusively  proved  that  every  motion 
made  by  animals,  every  bit  of  heat  a/ising  in  them, 
is  simply  a  portion  of  the  energy  which  this  world 
has  received  from  the  sun.  The  process  of  its  trans- 
formation is  as  follows : 

Plants,  by  virtue  of  the  possession  of  a  body  called 
chlorophyll  (i.e.,  their  green  coloring  matter),  have  the 
power  of  using  the  energy  of  sunlight.  By  means  of 


1 8  THE  LIVING  WORLD. 

the  energy  thus  within  their  reach,  they  are  able  to 
build  complicated  chemical  substances  out  of  such 
simple  compounds  as  water,  carbonic  acid,  and  simple 

^nitrogenous  compounds.  It  is  a  well-known  principle 
of  physics  that  to  build  a  complicated  chemical  body 
out  of  simple  ones  requires  the  exertion  of  energy, 
just  as  it  does  to  place  a  number  of  bricks  one  on  top 

^of  another.  All  of  the  energy  thus  used  is  rendered 
latent,  but  it  can  all  be  obtained  again  in  active  con- 
dition by  pulling  the  structure  to  pieces.  Every  com- 
plex chemical  compound  may  therefore  be  looked 
upon  as  a  store  of  energy.  Plants,  then,  growing  in 

/The  sunlight  are  continually  making  use  of  the  sun's 
rays  to  enable  them  to  build  up  complex  compounds, 
and  they  are  therefore  storing  up  the  sun's  energy 
in  the  form  of  chemical  energy.  The  energy  of  their 
life  therefore,  consists  in  transformed  sunlight.  Now 
animals  use  as  food  the  chemical  compounds  thus 
built  by  plants.  Animals,  unlike  plants,  are  not  able 
to  make  use  of  the  sun's  rays  directly,  but  they  can 
make  use  of  the  store  of  energy  provided  by  the 
plants.  They  therefore  derive  all  the  energy  of  their 
life  by  breaking  to  pieces  these  products  of  the 
plant's  constructive  power.  Just  as  the  steam-engine, 
by  breaking  to  pieces  the  coal  which  forms  its  fuel, 
makes  use  of  the  energy  thus  liberated,  so  the  body 
by  similarly  breaking  to  pieces  its  food  makes  use  of 

ithe  energy  thus  liberated.  The  steam-engine  con- 
erts  the  energy  of  chemical  composition  contained 
in  its  coal  into  motion  and  heat ;  the  body  also  con. 
verts  the  energy  of  chemical  composition  contained 
in  its  foods  into  motion  and  heat.  All  of  this  is 


vi1 

V 


THE   ORIGIN   OF  LIFE.  lg 

practically    granted    everywhere,   and    we   need   not 
attempt  to  question  the  conclusion  further.     All  of 1 
the  energy   of  the   body  is  a  part  of   the   physical 
energy  of  the  universe,  and  its  forces  are  correlated 
with  other  physical  forces. 

Relation  of  Vital  Activities  to  Chemical  Laws. 

Again,  it  will  hardly  be  questioned  to-day  that  the  / 
chemical  processes  going  on  in  the  living  body  are 
fundamentally  similar  to  those  which  may  take 
place  out  of  the  body.  The  same  laws  of  chemical 
affinity  govern  the  changes  taking  place  in  the  body 
and  those  occurring  in  experiments  in  the  laboratory. 
The  chemical  processes  of  the  body  may  be  con- 
sidered under  two  classes.  The  first  are  the  processes 
of  construction,  by  which  simple  bodies  are  built  into 
complex  ones.  This  class  is  chiefly  found  in  plants. 
The  second  are  those  of  destruction,  by  which  the 
complex  bodies  are  broken  into  simpler  ones.  This 
class  is  chiefly  characteristic  of  animals,  though 
found  also  in  plants.  The  destructive  changes  are 
the  simpler,  and  there  is  no  reason  to  think  they  are 
any  different  from  the  destructive  processes  of  the 
laboratory.  The  essential  feature  is  oxidation,  ancl^ 
oxidation  may  take  place  anywhere.  It  is  true  that 
the  details  of  the  process  of  this  destruction  in 
organic  beings  differs  in  some  respects  from  that 
which  chemists  have  been  able  to  simulate.  WherT) 
food  is  thus  broken  up  in  organisms,  many  decom- 
position products  arise  which  do  not  occur  when  the 
process  is  carried  on  in  the  laboratory.  These  prod- 
ucts are  thus  characteristic  of  organic  beings,  and  it 


2O  THE   LIVING  WORLD. 

was  for  a  long  time  believed  that  they  could  never 
be  obtained  except  through  the  influence  of  vitality. 
Modern  chemistry  has  demonstrated  the  possibility 
of  making  many  of  them  in  the  laboratory ;  many  of 
the  simpler  ones  have  already  been  manufactured, 
and  the  list  is  constantly  increasing. 

Plainly,  however,  the  destructive  processes  are  by 
no  means  so  important  as  the  constructive  ones.  In 
plants  the  simplest  compounds,  H2O,  CO2,  and 
NH3  are  built  under  the  influence  of  sunlight,  into 
the  most  complicated  ones.  Even  in  animals  this 
constructive  power  is  essential,  for  they  do  not  con- 
tent themselves  with  simply  pulling  to  pieces  the 
products  of  the  life  of  plants.  They  do  destroy  most 
of  them,  but  the  energy  liberated  enables  them  to  do 
a  certain  amount  of  building  for  themselves.  They 
change  dead  matter  into  living  matter,  which  must 
be  looked  upon  as  a  constructive  process.  Now  our 
chemists  tell  us  that  they  have  reason  for  believing 
that  even  these  constructive  processes  are  purely 
chemical,  and  will  one  day  be  simulated  in  the  lab- 
oratory. They  have,  indeed,  already  shown  that 
many  of  these  organic  bodies  can  be  manufactured 
synthetically.  Plants  manufacture  protoplasm,  the 
most  complicated  body  of  which  we  have  any 
knowledge.  By  the  decomposition  of  this  body  may 
be  obtained  a  long  series  of  decomposition  products, 
which  become  simpler  and  simpler  until  they  are 
once  more  resolved  into  the  simple  ones  with  which 
the  plant  started.  Now  our  chemists  have  begun 
with  these  simple  bodies,  CO2,  H2O,  NH3,  etc., 
and  have  begun  to  climb  this  ladder  of  compounds 


THE   ORIGIN  OF  LIFE.  21 

toward  protoplasm,  To  be  sure,  they  have  not  yet 
climbed  very  far,  but  they  have  made  some  advance. 
Many  of  the  simpler  members  of  the  series  have 
been  manufactured  synthetically  from  simple  inor-. 
ganic  compounds.  And  since  they  have  truly  begun*"* 
to  ascend  through  this  series,  it  is,  of  course,  an  easy 
inference  to  predict  that  they  will  some  day  reach 
the  top  and  be  able  to  make  the  higher  members, 
even  protoplasm  itself.  In  theory  they  already  tell 
us  what  the  albumens  are  chemically,  and  expect  to 
be  able  to  make  them  synthetically  before  a  great 
while ;  indeed,  at  the  present  time  chemists  are 
startled  by  the  recent  announcement  of  the  manu- 
facture of  a  proteid  in  the  laboratory  of  Schutzen- 
berger.  Scientists  are  thus  looking  forward  to  the 
time  when  they  will  be  able  to  make  protoplasm  in 
the  laboratory,  and  thus  artificially  to  make  living 
things.  Judging  from  the  general  tendency  of  ad- 
vance, it  does  not  perhaps  seem  improbable  that 
they  may  some  time  make  a  body  which  shall 
have  the  chemical  composition  of  protoplasm  ;  but 
whether  or  not  this  body  would  be  alive  is  the  very 
question  at  issue. 

Of  course  it  is  perfectly  evident  that  the  methods 
used  by  the  chemist  in  these  syntheses  are  very 
different  from  those  employed  by  living  cells.  The 
chemist  uses  complicated  apparatus  and  long,  round- 
about processes  to  produce  the  simple  organic  com- 
pounds. A  transparent  and  seemingly  structureless 
mass  of  protoplasm  builds  directly  and  with  ease  the 
most  complicated  bodies.  No  one  will,  of  course, 
pretend  to  compare  the  organic  cell  with  the  chem- 


22  THE  LIVING  WORLD. 

ical  laboratory  in  any  exact  sense.  vAll  that  our 
chemists  can  succeed  in  showing  is  that  organic 
changes  are  governed  by  the  same  chemical  laws  as 
those  which  regulate  inorganic  changes.  And  the 
possibility  of  the  manufacture  by  synthetical  pro- 
cesses of  a  number  of  the  simpler  organic  compounds 
gives  us  undeniable  evidence  that  chemical  laws  are 
the  same  in  the  body  and  in  the  laboratory. 

Properties  of  Life  as  Explained  by  Physical  and 
Chemical  Laws. 

Recognizing,  then,  that  all  the  energy  of  organ- 
isms is  derived  from  solar  energy,  and  that  the 
chemical  processes  in  the  body  are  essentially  sim- 
ilar to  those  outside,  the  next  question  to  be 
answered  is  how  far  the  vital  manifestations  of 
organisms  can  be  explained  according  to  these  laws ; 
to  see  whether  or  not  all  of  the  activities  of  living 
things  can  be  explained  by  physical  laws.  And 
here  too  when  we  reach  the  real  opinion  of  various 
thinkers,  we  find  something  like  unanimity  in  many 
points  at  least.  Understanding  the  doctrine  of  the 
conservation  of  energy,  it  is  at  once  evident  that  all 
of  the  energy  displayed  by  organisms  must  be 
transformed  solar  energy  ;  and  hence  all  of  the  mani- 
festations of  the  body  which  are  measurable  by 
units  used  in  measuring  other  physical  forces  must 
come  under  this  head.  Here  will  of  course  be  in- 
cluded all  the  forms  of  motion,  both  molar  and 
^molecular.  The  motions  of  the  body,  the  heat  of 
the  body,  expansion  and  contraction  of  protoplasm, 
all  electrical  phenomena,  probably  also  nervous  im- 


THE   ORIGIN  OF  LIFE.  2$ 


pulses — all  doubtless  come  in  this  category.  Indeed, 
all  manifestations  of  the  body  which  can  be  matched 
by  any  machine  will  unhesitatingly  be  set  down  as 
coming  under  the  head  of  transformed  physical 
forces.  We  can  believe  that  the  body  will  do  any- 
thing that  a  machine  can  do  without  calling  in  the 
aid  of  any  distinct  force.  In  other  words,  the 
activities  of  living  things,  though  more  complex,  are 
as  truly  due  to  physical  and  chemical  forces  as  those 
of  a  machine. 

It  is  when  we  come  to  the  other  properties  of  life 
not  found  in  machines  that  the  problem  becomes 
more  difficult.  No  machine  has  the  power  to  assimi- 
late food  and  grow.  These  properties,  which  really 
are  one,  form  the  second  character  which  universally 
distinguishes  living  matter  from  non-living  matter. 
Now,  so  far  as  the  mechanical  process  of  growth  is 
concerned,  it  is  simply  chemical  change.  This  is  cer- 
tainly so  in  animals.  They  take  into  their  body  certain 
complex  substances  as  food.  This  food  undergoes 
chemical  changes,  chiefly  those  of  oxidation.  As  a  re- 
sult, decomposition  products  are  obtained,  and  some 
of  these  products  of  decomposition  are  ejected,  while 
others  are  retained  in  the  body,  and  thus  the  body 
grows.  But  it  is  not  quite  so  simple  as  this,  for,  as 
we  have  seen,  a  certain  part  of  the  changes  are  con- 
structive. Some  of  the  decomposition  products  of 
the  food  are  united  together  into  more  complex 
compounds,  and  all  of  it  is  more  or  less  altered,  so 
that  none  of  the  food  is  retained  in  the  body  in 
exactly  the  same  condition  in  which  it  was  taken. 
In  short,  the  body  assimilates  its  food,  converting  it 


24  THE  LIVING  WORLD. 

from  its  dead  condition  into  its  own  substance.    Now 

•^—x* 

it  is  not  possible  to  imitate  many  of  these  changes  as 
yet  in  our  laboratories,  but  that  they  are  all  chemical 
changes  can  scarcely  be  questioned.  Even  the  con- 
structive changes  by  which  the  body  raises  the 
compounds  into  a  plane  of  greater  complexity,  must 
be  regarded  simply  as  chemical  processes,  the  energy 
which  is  required  for  them  being  obtained  by  the 
breaking  down  of  other  portions  of  food.  And  in 
plants  also  growth  must  be  regarded  as  a  chemical 
process,  for  it  consists  in  the  combination  of  simple 
organic  compounds  to  form  complex  ones  under  the 
influence  of  sunlight. 

The  third  property  of  living  matter — reproduction 
— seems  at  first  sight  to  be  a  more  marvellous  power 
than  that  of  growth  ;  but  most  biologists  think  that 
it  is  easily  derived  from  the  latter.  Fundamentally, 
reproduction  is  a  direct  and  necessary  result  of 
growth.  In  its  simplest  form,  as  found  in  the  uni- 
cellular animals,  it  is  seen  to  be  nothing  more  than  a 
division.  The  unicellular  organism  by  chemical  pro- 
cesses continues  to  assimilate  food  and  thus  to  grow. 
It  keeps  on  increasing  in  size  until  it  finally  becomes 
so  large  that  the  cohesion  between  its  parts  is  in- 
sufficient to  keep  the  great  bulk  together,  and  as  a 
result  it  divides  into  two  parts.  Each  of  these  parts 
is  of  course  like  the  other,  and  there  are  thus  two 
organisms  where  there  was  only  one  before.  This  is 
the  simplest  case  of  reproduction,  and  heredity  in 
this  case  is  to  be  explained  as  a  necessary  result  of 
growth.  Now,  it  is  not  difficult  to  see  how  the  more 
complex  forms  of  reproduction  may  have  been  de- 


THE   ORIGIN   OF  LIFE.  2$ 

rived  from  this.  If  instead  of  a  single  cell  there  were 
a  large  number  of  cells  attached  together,  growth 
might  lead  them  all  to  divide  in  a  similar  manner 
after  the  cohesion  of  parts  had  ceased  to  be  sufficient 
to  keep  them  together.  And  such  a  method  of  re- 
production docs  occur  in  large  groups  of  organ- 
isms. Or  it  might  be  that  certain  parts,  perhaps 
a  single  cell  only,  would  undergo  this  division,  the 
parts  of  this  cell  becoming  free  to  form  new  indi- 
viduals, and  thus  spores  would  arise.  Perhaps  two 
individuals  might  fuse  into  one,  and  thus  the  vigor 
of  both  would  be  combined  into  one.  This  would 
lead  to  sexual  reproduction.  And  so  on.  Of  course 
the  details  of  this  process  are  purely  hypothetical, 
and  it  is  not  our  purpose  to  dwell  upon  them  ;  but 
it  is  easy  to  see  that  reproduction  can  probably  be 
explained  upon  a  mechanical  basis  as  the  result  of 
assimilation  and  growth. 

It  is  thus  seen  that  the  three  properties  of  life  can 
receive  at  least  a  provisional  explanation  from  a  me- 
chanical standpoint  in  accordance  with  the  laws  of 
chemistry  and  physics,  and  since  we  are  at  present 
dealing  with  life  in  its  simplest  form,  we  need  not 
here  trouble  ourselves  with  its  higher  properties  of 
consciousness  and  intelligence.  The  explanations 
thus  offered  are  accepted  with  practical  universality 
by  biologists,  and  we  may  regard  the  preliminary 
question  in  the  problem  of  life  as  definitely  settled. 

Difference  between  the  Dead  and  Living  Organism. 

With  all  of  this  explanation  and  reduction  of  vital 
manifestations  to  physical  laws,  no  one  can  fail  to 


26  THE  LIVING  WORLD. 

realize  that  something'  is  lacking ;  and  that  though 
scientists  have  explained  much  by  hypothesis,  they 
have  yet  left  the  real  question  untouched.  It  is  not 
easy  to  determine  definitely  this  life  factor, — a  factor 
so  prominent  in  the  minds  of  those  who  disbelieve  in 
the  mechanical  theory  of  life,  and  so  readily  ignored 
by  those  who  hold  this  theory.  That  there  is  some- 
thing more  than  has  been  reached  by  this  explana- 
tion may  perhaps  be  made  evident  by  a  further 
consideration  of  the  parallel  between  an  organism 
and  a  machine.  The  comparison  between  the  dead 
body  and  the  machine  is  exact,  for  each  has  the 
mechanism  which  will  enable  it  to  transform  one 
sort  of  energy  into  another  under  the  right  con- 
ditions. But  in  the  body  the  requisite  condition  is 
the  presence  of  life,  whatever  that  may  be,  which 
guides  the  chemical  changes  taking  place.  In  the 
machine  the  necessary  condition  is  the  presence  of 
an  engineer,  who  guides  the  forces  and  chemical 
changes.  The  comparison  of  the  living  body  should 
not  be  simply  with  the  machine  in  motion,  but  with 
the  machine  plus  the  engineer.  This  difference  is 
great  indeed.  A  machine  may  be  ever  so  perfect, 
and  yet  will  not  perform  its  work  unless  its  engineer 
supply  its  proper  conditions.  Food  out  of  the  body 
will  never  go  through  the  complicated  changes  above 
mentioned  unless  subjected  to  very  peculiar  con- 
ditions by  the  chemist.  Food  in  the  body  will  not 
go  through  these  changes  unless  subjected  to  the 
action  of  life.  Sunlight  may  fall  upon  CO2,  H2O, 
and  NH3  eternally  without  producing  the  slightest 
tendency  toward  a  synthesis  of  these  elements.  But 


THE   ORIGIN"  OF  LIFE.  2/ 

let  this  occur  in  any  living  green  plant,  and  how  dif- 
ferent the  result.  In  some  way  living  matter  causes 
a  synthesis  to  take  place.  The  presence  of  life  in  an 
organism  causes  certain  chemical  changes  to  be  set 
up  in  it  which  result  in  growth.  Remembering  that 
none  of  these  changes  will  take  place  of  their  own 
accord,  it  is  perfectly  evident  that  there  is  some- 
thing in  the  organism  beyond  simple  chemical 
affinity, — some  sort  of  power  which  directs  chemical 
changes.  Whatever  it  may  be,  it  is  the  essence  of 
life.  In  almost  every  sentence  used  in  the  com- 
parison of  animals  with  machines  this  factor  can  be 
seen.  Even  Huxley,  the  foremost  in  the  mechani- 
cal theory,  says,  "  We  touch  the  spring  of  the  word 
machine,"  and  the  result  is  speech,  and  the  term 
"  we  "  implies  something  not  present  in  machines. 

That  there  is  a  difference  between  organisms  and 
machines  at  this  point  may  be  made  more  evident  by 
consideration  of  the  difference  between  living  and 
dead  organisms.  That  the  body  is  a  machine,  and 
that  like  the  machine  it  converts  chemical  energy 
into  mechanical  energy  will  to-day  be  everywhere 
admitted.  But  a  machine  cannot  die.  A  machine 
may  stop  its  motions,  but  a  machine  at  rest  is  not 
comparable  with  the  dead  body.  In  both  cases,  it  is 
true,  there  is  a  cessation  of  the  changes  which  con- 
stitute activity,  but  in  the  one  case  the  changes  may 
be  resumed  again,  in  the  other  this  is  impossible. 
A  dead  body  can  never  be  revivified.  It  is  more 
strictly  to  be  compared  to  a  machine  which  has  lost 
its  engineer,  for  with  this  loss  disappears  all  possi- 
bility of  further  action.  Its  mechanism  may  be 


28  THE  LIVING  WORLD. 

perfect,  and  it  may  have  all  the  possibilities  of  fur- 
ther action  except  a  directing  power,  but  without 
this  it  is  forever  quiet.  And  so  an  organism  is,  so 
far  as  we  can  see,  frequently  intact  after  death,  with 
all  of  its  mechanism  present ;  there  is  just  as  much 
stored  energy  in  a  pound  of  fat  in  the  dead  body  as 
in  the  living  body,  L  id  it  is  just  as  capable  of  being 
oxidized.  So  far  as  we  can  see,  therefore,  every 
physical  condition  may  be  present  in  the  dead  body 
which  is  necessary  to  produce  the  process  known  as 
life,  if  the  process  could  once  be  started.  But  with- 
out this  spark  of  life  to  start  and  direct  the  chemical 
changes  no  life  can  show  itself.  And  in  like  manner 
do  we  find  all  other  comparisons  ever  made  between 
organic  and  inorganic  matter  failing  at  this  point. 
Living  things  have  been  compared  with  crystals,  for 
both  grow,  although  the  process  of  growth  is  very 
different  in  the  two  cases.  But  a  crystal  cannot  die. 
Take  it  out  of  the  solution  upon  which  its  growth 
depends  and  it  will  cease  to  grow,  forever  remaining 
stationary.  Put  it  back  in  the  solution,  and  once 
more  it  will  resume  its  growth.  A  steel  bar  may  be 
magnetized,  and  under  these  conditions  will  exhibit 
properties  which  it  did  not  possess  before.  But  it 
may  be  demagnetized  by  a  blow,  and  thus  lose  all  of 
these  properties.  This  seems  indeed  to  bear  much 
resemblance  to  death  until  we  remember  that  a  steel 
bar  may  be  magnetized  and  demagnetized  indefinitely, 
and  never  once  fail  to  exhibit  its  properties.  But  an 
organism,  when  it  has  once  lost  its  vitality,  can  never 
be  brought  to  assume  a  vital  condition.  It  is  perfectly 
plain  that  at  this  point  all  of  the  comparisons  of 


THE   ORIGIN   OF  LIFE.  2g 

organic  with  inorganic  processes  fail.  There  are 
some  conditions  supplied  by  the  living  organism  not 
found  in  the  inorganic  world,  and  these  conditions, 
whatever  they  may  be,  direct  the  play  of  chemical 
forces  in  the  organism. 

The   Vitalistic   Theory. 

We  have  now  finally  reached  the  question  at  issue. 
The  vitalistic  question  to-day  is  not  to  decide  how 
many  activities  of  organism  can  be  explained  by 
chemical  and  physical  laws,  but  to  discover  what  are 
the  conditions  which  regulate  these  processes;  to 
decide  why  it  is  that  a  living  body  can  induce 
chemical  changes  which  are  impossible  in  the  dead 
body. 

One  answer  which  has  long  been  given  to  this 
question  is  that  the  necessary  condition  is  the  pres- 
ence of  a  "  vital  force  "  ;  a  force  uncorrelated  with 
other  forces, — a  distinct  entity  in  itself.  This  force 
is  life.  It  is  conceived  as  having  been  supplied  to 
the  world  at  the  beginning  of  life  on  the  globe,  and 
as  having  been  handed  down  from  one  generation 
to  another,  or  perhaps  created  anew  at  each  birth. 
Vitality  is  therefore  considered  as  something  apart 
from  the  physical  universe,  but  as  capable  of  exert- 
ing an  influence  upon  matter,  to  direct  the  changes 
taking  place  in  it.  According  to  this  view,  spon- 
taneous generation  would  be  an  impossibility,  for  this 
vital  force,  not  being  derivable  from  other  forces, 
could  have  its  origin  only  from  previously  existing 
vital  force.  This  theory  labors  under  the  disadvan- 
tage of  being  unable  to  say  what  is  meant  by 


30  THE  LIVING  WORLD. 

vital  force,  for  of  course  we  can  get  .no  conception 
of  any  force  except  by  its  results.  But  the  vitalistic 
theory  claims  that  life  is  an  immaterial  something 
which  directs  physical  processes  so  as  to  produce  the 
activities  which  distinguish  living  things.  We  need 
not  further  consider  this  view,  for  it  consists  chiefly 
in  recognizing  the  necessity  of  something  more  than 
chemical  affinity  and  change,  and  in  acknowledging 
our  inability  to  explain  it  by  giving  to  it  the  name 
vitality. 

The  Mechanical  Theory  of  Life. 

The  point  of  dispute  to-day  is  not  whether  the 
vitalistic  theory  would  explain  facts,  but  whether  it 
is  necessary.  A  purely  mechanical  view  of  life  has 
slowly  arisen  from  the  profuse  speculations,  which 
claims  to  be  able  to  meet  the  case  without  recourse 
to  any  imaginary  "  vital  force."  The  general  ten- 
dency of  scientific  thought  gives  a  certain  amount 
of  a  priori  bias  in  favor  of  such  a  view.  It  has 
unquestionably  been  the  tendency  of  science  to 
explain  more  and  more  of  the  phenomena  of  the 
world  in  terms  of  the  properties  and  laws  of  the 
natural  universe.  The  foundation  of  the  law  of  the 
conservation  of  energy,  the  conception  of  forces  as 
modes  of  motions,  are  great  steps  in  this  direction. 
In  the  organic  world  the  theory  of  evolution,  the 
application  of  the  conservation  of  energy  to  the 
mechanics  of  life,  the  perception  that  the  same 
chemical  laws  govern  living  things  and  dead,  and 
every  discovery  of  likeness  between  vital  processes 
and  those  purely  mechanical,  are  all  steps  toward 


THE   ORIGIN  OF  LIFE.  31 

this  general  unification.  It  is  certainly  in  a  line  with 
this  advance  to  reach  a  mechanical  view  of  this  life 
essence.  If,  therefore,  a  mechanical  explanation  is 
possible,  there  is  good  reason  for  believing  that  it  is 
in  the  line  of  truth. 

The  mechanical  theory  is,  in  brief,  that  the  direc- 
tive conditions  of  which  we  are  in  search  are  simply 
those  of  chemical  composition  and  molecular  ar- 
rangement. It  is  pointed  out  that  the  properties 
of  compounds  increase  in  complexity  with  the  in- 
crease of  the  complexity  of  the  compounds,  that  as 
the  molecule  becomes  more  complicated  its  powers 
and  possibilities  become  more  diversified.  The  prop- 
erties of  compounds  have,  moreover,  no  traceable 
relation  to  the  properties  of  the  elements  from  which 
they  are  made.  Oxygen  and  hydrogen,  when  they 
unite,  form  water,  a  compound  with  properties  not 
possessed  by  either  of  the  elements  ;  and  yet  we  do 
not  doubt  that  they  are  due  to  the  properties  of  the 
elements.  It  is  therefore  easy  to  make  the  far- 
reaching  assumption  that,-  when  the  molecule  be- 
comes as  complicated  as  that  of  protoplasm,  its 
properties  will  be  as  complicated  as  those  of  living 
things.  One  of  these  properties  is  to  induce  chemi- 
cal changes  in  foods.  Just  as  it  is  the  property 
of  water  to  dissolve  many  chemical  substances,  so 
it  is  the  property  of  the  highly  complex  body  pro- 
toplasm to  cause  chemical  changes.  When  it  is 
possible,  we  are  told,  to  manufacture  the  chemical 
substance  protoplasm,  it  will  of  necessity  be  alive, 
for  there  are  no  peculiar  powers  in  organisms  not 
inherent  in  them  as  the  result  of  molecular  arrange- 


32  THE  LIVING  WORLD. 

ment.  The  directive  power  which  seems  to  exist  is 
no  directive  power  at  all,  but  only  a  property  of 
protoplasm.  Just  as  it  is  the  property  of  platinum 
sponge  to  cause,  when  held  in  a  current  of  hydro- 
gen, the  hydrogen  to  unite  with  oxygen  and  burn, 
so  it  is  the  property  of  protoplasm  to  cause  more 
complicated  oxidations  to  take  place,  which  produce 
the  fundamental  process  of  growth,  and  from  this, 
as  we  have  seen,  other  vital  activities  easily  follow. 
We  see,  therefore,  that  the  comparison  of  the  body 
with  the  machine  plus  its  engineer  is  replaced  by 
a  machine  that  is  purely  automatic,  and  finds  in  its 
own  complex  composition  the  conditions  which 
regulate  its  activities.  Death,  according  to  this 
idea,  is  simply  the  destruction  of  protoplasm,  which 
would,  of  course,  destroy  its  properties.  Just  as 
soon  as  protoplasm  begins  to  lose  its  complicated 
structure,  it  loses  all  of  the  properties  belonging  to  it 
as  protoplasm  ;  and  this  is  death.  Demagnetism  of 
a  bar  of  steel  is  therefore  strictly  comparable  to 
death,  the  only  difference  being  that  it  is  possible  to 
cause  the  steel  bar  to  resume  its  former  molecular 
arrangement  and  once  more  to  possess  its  magnetic 
properties.  The  possibility,  however,  does  not  exist 
in  living  things;  the  violence  of  death  ruins  the 
machine.  Even  a  machine  cannot  be  started  if  its 
adjustments  are  broken,  and  a  living  body  being 
more  complex  than  any  machine  has  its  harmonious 
action  more  readily  ruined.  What  is  lost  in  death  is, 
therefore,  not  any  directing  force  but  chemical  or 
molecular  composition.  The  dead  body  is  to  be 
compared  not  with  a  machine  which  has  lost  its 


THE    ORIGIN  OF  LIFE.  33 

engineer,  but  with  a  broken  machine  which  cannot 
be  mended.  Life  is  thus  only  an  abstraction  from 
the  properties  of  living  things,  just  as  aquosity  would 
be  an  abstraction  from  the  properties  of  water. 

This  mechanical  theory  of  life  is  not  at  present 
open  to  direct  argument.  The  dynamics  of  proto- 
plasm may  be  studied  carefully  ;  it  may  perhaps  be 
shown  that  all  the  activities  of  protoplasm  are  easily 
explained  as  the  result  of  chemical  and  physical 
forces.  Already  scientists  are  beginning  to  compre- 
hend how  the  movements  of  protoplasm,  which  have 
proved  so  puzzling,  are  intelligible  as  the  results  of 
chemical  change,  whereby  the  density  of  the  sub- 
stance is  altered,  and  consequently  its  shape.  In- 
deed appearances  seem  to  indicate  that  perhaps  all 
the  activities  of  protoplasm  may  be  explained  thus 
easily.  But  all  of  this  fails  to  reach  the  real  ques- 
tion at  issue,  which  asks  for  the  directive  cause  of 
these  changes.  The  only  direct  argument  would 
be  to  manufacture  protoplasm  and  have  it  begin  to 
assimilate  food,  or  to  show  in  some  other  way  that 
a  purely  automatic  machine  is  a  possibility,  which 
shall,  as  organisms  do,  supply  itself  with  its  own  con- 
ditions of  activity.  Until  this  is  done  the  mechani- 
cal theory  can  only  be  an  inference  from  the  general 
tendency  of  scientific  advance. 

The  Origin  of  Life. 

It  is  very  plain  that  our  verdict  in  regard  to  the 
origin  of  life  in  the  world  will  depend  largely  upon 
what  position  we  assume  on  the  question  just  dis- 
cussed.    If  we  assume  that   life   is   a  distinct  force 
3 


34  THE  LIVING  WORLD. 

unrelated  to  other  forces  of  nature ;  if,  in  short,  we 
accept  the  vitalistic  standpoint,  the  matter  becomes 
very  simple.  Such  a  force  could  not  have  come  into 
existence  except  by  a  creative  fiat.  We  should 
simply  say  then  that  far  back  in  the  history  of  the 
world  some  supernatural  power  introduced  the  first 
germ  of  life  into  the  world,  and  that  this  first  germ 
was  the  simplest  form  of  protoplasm.  To  one  who 
holds  this  view  all  the  attempts  to  find  a  natural 
explanation  of  the  origin  of  life  are  useless. 

If,  however,  we  are  willing  to  accept,  even  pro- 
visionally, the  view  that  life  is  not  a  distinct  essence, 
but,  as  the  mechanical  theory  of  life  would  tell  us, 
simply  an  abstraction  from  the  complex  properties 
of  the  substance  protoplasm,  then  the  question  of  its 
origin  assumes  a  new  aspect.  It  becomes  then  a 
legitimate  question  to  ask  how  life  arose  in  the 
world.  Life  by  its  inherent  qualities  is  self-perpetu- 
ating, and  if  once  it  makes  its  appearance  in  the 
world  its  remaining  here  so  long  as  the  conditions 
admit  is  a  matter  of  course.  Geology  tells  us,  how- 
ever, that  at  one  time  the  earth  was  so  heated  that 
no  living  thing  could  have  existed  on  its  surface.  It 
follows  from  .this  fact,  that  life  on  the  globe  must 
have  had  a  beginning.  What  then  was  the  nature  of 
the  forces  which  brought  the  first  living  matter  into 
existence  ? 

Spontaneous  Generation, 

First  we  must  ask  if  experiment  or  observation 
gives  us  any  reason  for  believing  that  living  matter 
can  arise  from  that  which  is  not  living.  Ever  since 


THE   ORIGIN   OF  LIFE.  35 

living  matter  has  been  studied  it  has  been  believed 
by  many  that  living  organisms  could  arise  spon- 
taneously, i.e.j  without  having  any  direct  living 
ancestors.  Aristotle  held  this  view,  and  from  his 
time  for  centuries  no  one  presumed  to  doubt  that 
most  of  the  smaller  organisms  could,  and  usually  did, 
arise  spontaneously.  It  was  not  until  the  sixteenth 
century  that  the  matter  became  one  of  discussion. 
At  that  time  Redi  discovered  that  fly-maggots  were 
not  produced  spontaneously  from  decaying  flesh  as 
had  hitherto  been  believed,  but  came  from  some- 
thing deposited  by  adult  flies.  This  discovery  led 
him  to  further  observation,  and  finally  to  the  con- 
clusion that  there  was  no  such  thing  as  spontaneous 
generation.  Since  that  time  this  doctrine  has  been 
the  ground  of  many  a  hard-fought  battle.  The  fol- 
lowers of  Redi  speedily  began  to  show  by  careful 
study  of  the  facts  that  numerous  cases  of  so-called 
spontaneous  generation  were  simply  due  to  careless 
observation.  It  was  soon  proved  that  at  least  all 
the  higher  animals  arise  by  the  method  of  reproduc- 
tion only.  The  adherents  of  the  belief  that  life  can 
arise  from  the  non-living  were  thus  driven  to  base 
their  claims  upon  the  origin  of  the  smaller  organ- 
isms, and  finally  upon  microscopic  forms,  which 
can  be  studied  only  with  extreme  difficulty.  But 
so  far  from  admitting  this  to  be  a  retreat  from  their 
position,  they  have  shown  that  it  is  the  most  natural 
conclusion  possible.  For  a  priori  grounds  should 
serve  to  convince  us  that  if  living  things  can  arise 
from  the  non-living,  this  would  be  true  only  of  the 
very  lowest  organisms,  those  which  approach  the 


36  THE  LIVING  WORLD. 

nearest  to  the  condition  of  simple  protoplasm.  The 
disproof  of  the  claims  of  the  earliest  biologists  who 
believed  in  the  abiogenetic  origin  of  the  higher 
animals,  is  therefore  no  proof  or  even  indication 
that  this  does  not  occur  in  the  lowest  organisms. 
The  question,  therefore,  finally  settled  around  the 
origin  of  the  lowest  and  smallest  forms  of  life. 
From  this  point  the  matter  has  been  chiefly  one  of 
care  in  experimenting.  It  was  found  by  some  that 
low  organisms,  bacteria,  infusoria,  etc.,  would  arise 
in  closed  flasks  filled  with  various  material  for  food, 
even  after  all  apparent  precautions  had  been  taken 
to  exclude  everything  alive.  But  other  experiment- 
ers employing  greater  precautions  for  the  exclusion 
of  living  matter  obtained  opposite  results.  The  ver- 
dict vibrated  from  one  side  to  the  other,  as  different 
experiments  were  made  known,  until  at  length  Pas- 
teur and  Tyndall  showed  that  the  negative  conclu- 
sion was  the  only  tenable  one.  Tyndall,  more  espe- 
cially, by  a  series  of  careful  experiments  conducted 
in  a  manner  beyond  reach  of  criticism,  so  conclu- 
sively proved  that  with  the  proper  precautions  no 
living  organisms  could  arise  in  any  solution  without 
the  access  of  previously  living  organisms,  that  no 
one  has  seriously  questioned  the  matter  since.  This 
result,  indeed,  is  only  a  negative  one.  It  simply 
shows  that  no  living  organisms  did  arise  under  the 
conditions  of  the  experiment.  But  it  is  so  conclu- 
sive that  scientists  have,  with  practical  unanimity, 
given  up  all  claim  that  there  is  the  slightest  evidence 
for  the  possibility  of  spontaneous  generation.  And 
this  is  admitted  by  the  very  men  who  still  insist  that 


THE   ORIGIN  OF  LIFE.  37 

spontaneous  generation  must  have  occurred  at  some 
time  in  the  history  of  the  globe. 

While  it  is  thus  true  that  scientists  have  somewhat 
reluctantly  given  up  this  fascinating  theory,  it  by 
no  means  indicates  that  they  have  given  up  the  be- 
lief in  the  possibility  of  life  arising  from  the  non- 
living under  the  right  conditions.  Although  no  one 
has  as  yet  been  able  to  produce  conditions  under 
which  life  can  arise,  this  by  no  means  proves  that 
under  different  conditions  a  different  result  might 
not  be  reached.  Protoplasm  will  not  arise  in  closed 
flasks,  but  this  does  not  show  that  it  cannot  do  so 
at  the  bottom  of  the  sea.  If  it  could  be  shown  that 
life  arises  spontaneously  nowhere  on  the  globe  at  the 
present  time,  this  would  by  no  means  prove  that  in 
other  ages,  under  different  conditions,  it  may  not  so 
have  arisen.  And,  indeed,  now  that  the  possibility 
of  spontaneous  generation  to-day  is  practically  de- 
cided in  the  negative,  it  is  beginning  to  be  recognized 
that  the  experiments  thus  far  are  utterly  futile  to 
settle  the  primary  question  at  issue.  Even  if  a 
positive  result  had  been  obtained,  it  would  have  had 
scarcely  any  bearing  upon  the  question  of  the  origi- 
nal appearance  of  life.  This  will  be  evident  from 
the  following  considerations.  The  first  living  things 
must  have  been  able  to  make  use  of  inorganic  ma- 
terial for  food,  since  there  could  of  course  have  been 
no  organic  food  existing  at  that  time.  Our  experi- 
menters on  spontaneous  generation  have,  however, 
always  used  organic  solutions  in  their  experiments. 
Now,  to-day  only  organisms  which  are  supplied  with 
chlorophyll  are  able  to  raise  inorganic  matter  into  an 


38  THE  LIVING    WORLD. 

organized  condition.  At  the  present  time,  at  least,  all 
organic  life  depends  upon  the  action  of  chlorophyll. 
But  in  the  experiments  upon  spontaneous  generation 
it  has  only  been  claimed  that  such  organisms  as  bac- 
teria and  infusoria  could  arise  spontaneously  ;  and 
these  organisms  containing  no  chlorophyll  have  no 
power  to  live  upon  the  inorganic  world.  Our  ex- 
perimenters have  found  it  necessary  to  supply  them 
with  an  abundance  of  organic  food.  Such  organisms 
certainly  could  not  have  been  the  first  ones  to  appear 
upon  the  earth,  since  they  would  be  capable  of  exist- 
ing only  so  long  as  organic  food  was  supplied  to  them. 
Indeed,  if  we  could  imagine  the  ocean  filled  with 
albuminous  food  before  any  life  appeared,  and  then 
assume  that  these  organisms  could  arise  spontane- 
ously, we  should  be  no  nearer  to  a  permanent  origin 
of  life  than  we  were  before.  The  only  result  would 
be  a  rapid  multiplication  of  these  bacteria  until  the 
ocean  was  filled  with  them  ;  the  food  would  be  con- 
sumed, and  then  all  would  die  of  starvation,  since 
they  would  be  unable  to  make  food  for  themselves 
out  of  the  inorganic  world  as  green  plants  can  do. 
The  first  living  things  must  have  been  able  to  make 
use  of  the  inorganic  world,  and  plainly,  so  long  as 
experiments  deal  only  with  chlorophylless  organisms 
arising  in  organic  solutions,  they  have  no  direct  rela- 
tion to  the  question  of  the  primary  origin  of  life. 

Should  we  then  place  the  origin  of  life  in  the  same 
category  of  insolvable  mysteries  as  the  origin  of  the 
universe  in  general?  Looking  at  the  universe  in  the 
most  extreme  mechanical  manner  it  is  impossible  to 
think  of  it  without  some  original  creative  power. 


THE   ORIGIN  OF  LIFE.  39 

Behind  the  whole  we  must  posit  something  which 
no  thought  can  comprehend.  If  we  must  find  crea- 
tive power  somewhere,  perhaps  the  beginning  of  life 
may  be  an  instance  of  its  action.  It  may  be  well 
then,  inasmuch  as  it  seems  probable  that  the  origin 
of  life  can  be  nothing  but  a  matter  of  speculation,  to 
class  it  with  the  origin  of  matter  and  force,  and  thus 
to  cease  to  explain  it. 

The  question,  then,  stands  something  like  this. 
There  is  not  the  slightest  evidence  to-day  for  be- 
lieving that  life  can  arise  in  any  other  way  than 
through  the  influence  of  other  living  protoplasm. 
From  the  earliest  living  thing  to  the  present  there 
seems  to  have  been  a  direct  continuity  of  protoplasm, 
generation  after  generation  resulting  from  the  normal 
processes  of  reproduction,  but  in  no  other  way.  May 
it  not  be  best  to  abandon  the  question  of  the  origin 
of  life  and  to  say  that  it  first  appeared  as  the  result 
of  a  creative  fiat  ? 

But  this  science  refuses  to  do.  Science  grants 
that  there  are  insolvable  mysteries,  and  that  the 
mechanical  conceptions  of  the  universe  cannot  ex- 
plain all  things.  The  origin  of  matter  and  force,  the 
origin  of  motion  and  consciousness,  are  utterly  in- 
solvable  mysteries,  and  are  hence  outside  the  realm 
of  science.  But  it  is  thought  that  the  origin  of  life 
is  not  one  of  the  transcendent  mysteries,  but  is  one 
which  will  in  due  time  be  solved.  This  belief  has 
been  more  especially  prevalent  among  scientists 
since  the  precipitate  advance  of  speculation  in  the 
last  twenty-five  years,  due  to  the  growth  of  the  ideas 
comprised  in  the  theory  of  evolution.  This  theory 


4O  THE  LIVING  WORLD. 

or  group  of  theories  has  led  to  a  belief  in  the  general 
efficiency  of  natural  law  to  account  for  natural  phe- 
nomena ;  and  from  this  conception  has  arisen  the 
claim  that  there  must  have  been  a  natural  origin  of 
life.  While  then  biologists  have  somewhat  reluc- 
tantly given  up  their  beliefs  in  the  present  possibility 
of  spontaneous  generation,  many  of  them  even  the 
more  strenuously  assert  that  at  some  time,  in  some 
way,  life  must  have  arisen  from  the  non-living. 

Speculations  as  to  a  Mechanical  Origin  of  Life. 

Unable,  therefore,  to  obtain  direct  evidence  either 
for  or  against  its  proposition  of  a  natural  origin  of 
life,  science  endeavors  to  meet  the  question  by 
speculation.  Having  shown  that  vital  processes  are 
closely  related  to  chemical  and  physical  conditions, 
suggestions  as  to  a  possible  causal  connection  be- 
tween the  two  are  of  some  significance.  Speculations 
as  to  the  origin  of  life  can,  therefore,  hardly  be 
called  absurd,  though  they  are  almost  unfounded  in 
fact.  Although  they  cannot  be  regarded  as  having 
much  value,  nevertheless  modern  scientific  beliefs 
are  in  a  measure  founded  on  them. 

We  may  pass  over  as  irrelevant  the  suggestion 
that  life  may  have  been  brought  into  the  world  by 
meteors.  This  does  not  of  course  assist  in  the  slight- 
est degree  in  solving  the  question  of  the  origin  of 
life.  While  different  thinkers  will  hold  different 
views  upon  the  general  question,  nearly  all  will  de- 
pend upon  unknown  conditions  of  the  past  for  aid. 
A  line  of  speculation  something  like  the  following 
would  probably  not  be  far  from  expressing  the  gen- 


THE   ORIGIN   OF  LIFE.  41 

eral  outline  of  the  thoughts  of  most  biologists  to-day 
who  attempt  to  formulate  any  conception  of  the 
primal  origin  of  life  in  accordance  with  natural  law. 

Assuming  provisionally  that  life  is  simply  a  prop- 
erty of  the  complex  composition  of  protoplasm,  we 
can  go  on  to  ask  ourselves  how  this  complex  com- 
position could  ever  have  been  reached.  Now  certain 
facts  of  geology  assist  us  much  in  this  matter. 
During  the  early  history  of  the  globe  the  tempera- 
ture was  so  high  that  few,  if  any,  chemical  com- 
pounds could  exist.  As  the  earth  cooled  by  radiation, 
the  elements  hitherto  kept  apart  began  to  come  to- 
gether in  chemical  union.  All  during  the  long 
process  of  cooling  conditions  existed  which  have 
never  been  matched  since.  Even  after  the  tempera- 
ture had  reached  a  degree  which  admitted  the 
existence  of  organic  compounds,  every  circumstance 
was  utterly  different  from  what  is  found  to-day. 
Different  temperature,  different  relations  of  mois- 
ture, different  electrical  conditions,  an  atmosphere 
containing  vastly  more  carbonic  acid  and  oxygen 
than  ours  ;  all  these  factors,  and  thousands  of  others 
of  which  it  is  needless  to  speculate,  combined  to 
make  the  conditions  of  chemical  union  widely  differ- 
ent from  any  that  can  now  occur.  Under  these 
circumstances  it  is  plain  that,  with  the  universal 
chemical  laws,  chemical  processes  would  be  carried 
on  of  which  we  can  know  nothing,  but  which  would 
be  very  different  from  any  taking  place  in  the  world 
at  present,  or  which  can  be  simulated  in  the  labora- 
tory by  the  chemist  or  biologist.  In  these  early 
times  we  thus  see  the  possibility  of  production  of 


42  THE   LIVING  WORLD. 

an  almost  infinite  variety  of  compounds,  each  with 
its  own  peculiar  properties.  Some  of  these  com- 
pounds were  so  stable  as  to  continue  to  exist  down 
to  the  present  day,  almost  unchanged.  Others  were 
constantly  changing.  The  compounds  of  carbon 
especially  were  varied  and  unstable,  as  we  may  con- 
clude from  the  compounds  of  that  element  known 
to-day.  Many  of  these  carbon  compounds  doubtless 
would  disappear  with  a  change  of  conditions,  break- 
ing up,  to  enter  into  other  combinations  and  form 
other  unstable  compounds.  Now  amid  this  continued 
succession  of  changes,  the  conditions  of  heat,  elec- 
tricity, etc.,  might  at  one  time  have  been  such  as  to 
cause  the  elements,  carbon,  oxygen,  hydrogen,  and 
nitrogen,  all  of  which  were  present  in  the  atmosphere, 
to  unite  into  certain  complex  bodies  approximating 
organic  compounds.  That  this  is  a  possibility  be- 
comes evident  when  we  remember  that  our  chemists 
have  already  begun  to  make  these  elements  unite 
by  laboratory  methods.  Many  organic  compounds 
have  been  synthetically  manufactured  from  inor- 
ganic material.  Most  of  these  compounds  of  early 
times  probably  did  not  continue  to  exist  very  long, 
since  they  were  unstable,  and  had  no  power  of  self- 
preservation. 

'  Thus  far  perhaps  no  one  will  hesitate  to  follow 
the  scientist,  since  he  is  dealing  with  authentic  facts 
rather  than  with  speculation.  But  now  he  takes  a 
step  into  the  dark.  He  supposes  that  at  one  time 
these  elements  united  into  a  compound  which  was, 
owing  to  its  peculiar  composition,  capable  of  causing 
other  bodies  to  change.  By  virtue  of  this  power 


THE   ORIGIN  OF  LIFE.  43 

other  carbon  compounds  then  existing  were  caused 
to  assume  the  composition  of  the  new  one,  according 
to  the  laws  noticed  in  a  previous  section  of  this 
chapter.  Once  this  power  is  acquired  the  compound 
possessing  it  would  not  disappear  like  the  other 
unstable  compounds,  but  would  remain  permanent. 
For  this  substance  would  assimilate  food  and  grow, 
and  all  the  essential  features  of  life  can  be  deduced 
from  growth.  This  compound  was  of  course  proto- 
'plasm  in  its  simplest  form.  It  was  only  one  of  a 
large  number  of  complex  compounds,  which  made 
their  appearance  under  the  peculiar  chemical  condi- 
tions of  early  eras.  Numerous  others  were  doubt- 
less formed,  each  possessing  its  own  properties.  But 
only  that  compound  which  was  capable  of  assimila- 
tion could  continue  to  exist  in  an  active  condition 
during  the  subsequent  ages.  This  substance  event- 
ually absorbed  all  other  compounds  in  any  way 
similar  to  itself  which  may  have  arisen  contempo- 
raneously with  or  before  it,  and  it  remains,  there- 
fore, to-day  the  only  living  matter,  the  physical 
basis  of  life. 

It  will  be  seen  that,  according  to  this  speculation, 
the  first  form  of  living  matter  was  by  no  means 
similar  to  any  organism  of  to-day.  It  was  rather  a 
diffused  mass  of  protoplasmic  substance,  with  no 
differentiation  into  cells  or  parts  or  individuals.  It 
will  be  further  seen  that  it  is  not  necessary  to  as- 
sume that  this  first  protoplasm  possessed  chlorophyll 
as  has  been  claimed,  for,  according  to  the  hypothesis, 
there  were  many  other  carbon  compounds  of  high 
complexity  produced  at  the  same  time.  These  com- 


44  THE  LIVING  WORLD. 

pounds,  more  or  less  similar  to  protoplasm,  though 
not  capable  of  self-perpetuation,  would  serve  the 
first  protoplasm  as  food.  There  would  thus  be  no 
lack  of  organic  material  for  the  subsistence  of  life  of 
the  first  protoplasm,  even  though  this  first  organism 
were  incapable  of  feeding  upon  the  inorganic  world 
directly.  Doubtless,  too,  the  conditions  which  pro- 
duced the  first  living  protoplasm  existed  fcw-srlong 
time,  and  thus  living  matter  would  for  a  long  time 
be  brought  into  existence  by  processes  other  than 
those  of  reproduction.  Indeed,  there  was  no  defi- 
nite beginning  of  life.  Here,  as  elsewhere,  nature 
made  no  jump,  but  produced  life  as  she  produces 
everything  else,  by  slow  stages.  Chemical  processes 
of  early  times  resulted  in  the  production  of  many 
compounds  which,  acting  upon  each  other,  and  acted 
upon  by  the  changing  conditions,  became  modified 
in  an  infinite  variety  of  ways.  Their  complexity 
and  instability  became  very  great.  Finally,  some  of 
the  most  unstable  of  all  began  to  effect  changes  in 
others  which  resulted  in  assimilation,  and  thus  slowly 
the  properties  became  more  marked.  Simpler  and 
simpler  substances  were  made  use  of  as  food.  So 
long  as  the  original  conditions  lasted  there  would  of 
course  be  no  need  that  living  matter  should  possess 
the  properties  of  chlorophyll.  Nor  was  this  at  all 
necessary  while  circumstances  were  such  as  to  make 
possible  the  natural  development  of  high  carbon 
compounds.  Eventually  the  power  to  live  upon  the 
simpler  inorganic  foods  must  have  been  acquired. 
But  it  is  only  necessary  to  assume  that  this  power 
became  fully  developed  by  the  time  that  the  con- 


THE   ORIGIN  OF  LIFE.  45 

ditions  had  so  changed  that  protoplasm  could  no 
longer  be  developed  by  the  original  spontaneous 
method.  Perhaps  for  ages  protoplasm  existed  un- 
able to  use^organic  food,  but  finding  sufficient  food 
in  the  surrounding  complex  carbon  compounds. 
And  when  this  power  did  at  last  become  developed, 
it  was  not  acquired  by  all  protoplasm.  For  just  at 
this  point  the  organic  world  became  divided  into  two 
parts.  One  part  did  develop  chlorophyll,  and  has 
since  been  able  to  live  upon  inorganic  matter,  using 
the  energy  of  sunlight  to  build  this  matter  into  an 
organic  compound.  Finding  its  food,  carbonic  acid, 
water  and  nitrates  and  sunlight  everywhere,  this 
class  of  organisms  did  not  acquire  the  power  of 
motion.  The  other  half  of  the  living  world  never 
developing  chlorophyll  became  of  necessity  at  last 
parasitic  upon  the  plants,  and  developed  an  almost 
universal  power  of  motion  in  order  to  enable  it  to 
seek  food.  The  animal  and  vegetable  kingdoms  were 
thus  finally  separated  from  each  other,  with  the 
relations  which  they  hold  to-day. 

Such,  in  brief  outline,  is  the  substance  of  some  of 
the  modern  speculations  concerning  the  method  by 
which  life  arose.  It  represents  one  phase  of  such 
speculations,  and  is  subject  to  great  modification  in 
the  minds  of  different  thinkers.  It  is  plainly  open 
to  sufficient  criticism,  and  it  is  equally  clear  that  it 
is  not  capable  of  direct  proof,  at  least  in  the  present 
state  of  science.  It  is  as  moderate  in  its  terms  as 
any  of  the  suggestions  upon  the  subject,  and  makes 
as  slight  claims  upon  our  credulity.  It  will,  at  all 
events,  serve  our  present  purpose  of  giving  an  idea 


46  THE  LIVING  WORLD. 

of  the  relation  of  speculation  to  the  question  of  the 
origin  of  life. 

The  Significance  of  such  Speculations. 

Now  what  are  these  speculations  worth?  Many 
will  immediately  answer  that  they  are  worth  noth- 
ing. Others  may  regard  them  as  having  a  certain 
amount  of  suggestiveness,  but  no  great  value.  It  is 
perfectly  plain  to  every  one  that  they  are  purely 
hypothetical.  Not  only  are  they  unproven  hypoth- 
eses, but  they  are  further  of  such  a  nature  that 
there  can  be  no  evidence  either  for  or  against  them. 
They  must  unhesitatingly  be  set  down  as  scarcely 
more  than  bold  guesses  at  a  possibility.  Even  Hux- 
ley says :  "  Of  the  causes  which  have  led  to  the 
origination  of  living  matter  it  may  be  said  we  know 
almost  nothing."  If,  then,  science  is  to  confine  it- 
self to  facts,  these  suggestions  may  be  cast  aside  as 
worthless.  Why  is  it  then  that  we  find  so  many 
biologists  to-day  willing,  yes,  more  than  willing, 
anxious,  to  accept  them  ?  Certainly  it  is  not  because 
they  are  the  simplest  explanations,  not  because  a 
large  number  of  converging  lines  of  thought  point 
toward  them.  Those  who  seriously  discuss  these 
speculations,  or  regard  them  as  of  any  significance, 
do  so  from  some  cause  lying  outside  of  the  question 
itself. 

And  this  cause  is  to  be  found  in  certain  philosophical 
conceptions.  Science  studies  the  world  from  one 
standpoint  only  ;  a  standpoint  which  its  devotees 
naturally  believe  will  lead  them  most  surely  to  the 
truth.  This  study  of  nature  from  the  exterior  has  led 


THE   ORIGIN  OF  LIFE.  47 

to  the  grand  generalization  that  all  nature  is  gov- 
erned by  law.  The  significance  of  the  word  law  does 
not  particularly  concern  science,  but  is  left  to  other 
realms  of  thought.  Science  satisfies  itself  in  discov- 
ering and  applying  laws.  A  thorough  study  of  nature 
has  made  it  seem  probable  that  natural  law,  when 
thoroughly  comprehended,  will  explain  all  natural 
phenomena.  So  many  facts  formerly  relegated  to 
the  realm  of  the  supernatural  have  been  explained  by 
natural  law  that  science  has  determined  to  call  in  the 
supernatural  as  seldom  as  possible,  and  to  accept  no 
breaks  in  the  chain  of  law  unless  absolutely  forced  to 
do  so.  This  generalization  is  at  the  foundation  of  the 
terms — law  of  continuity  and  evolution,  as  they  are 
used  by  science  to-day.  The  significance  which  this 
question  of  the  origin  of  life  has  for  all  evolutionary 
theories  is  at  once  evident.  It  is  an  important  link 
in  the  chain  of  continuity,  for  unless  the  spontaneous 
generation  of  life  be  a  fact,  the  law  of  continuity  is 
no  law.  For  even  if  science  does  succeed  in  explain- 
ing the  development  of  life  from  the  lowest  to  the 
highest,  but  does  not  explain  the  origin  of  this  first 
form,  it  has  only  half  accomplished  that  for  which  it 
is  striving — viz. :  to  reduce  living  phenomena  to  the 
same  laws  which  govern  the  non-living.  It  is  not 
surprising,  therefore,  that  we  find  biologists  observ- 
ing, experimenting,  and  speculating,  in  order  to  find 
some  way  to  help  themselves  out  of  their  dilemma. 
To  any  one  who  is  inclined  to  believe  in  this  law  of 
continuity,  and  the  efficiency  of  natural  forces,  such 
speculations  as  the  above,  which  show  a  possible 
avoidance  of  a  break  at  the  beginning  of  life,  have  a 


48  THE  LIVING  WORLD. 

certain  amount  of  significance.  If  we  are  inclined  to 
believe  that  "  nature  does  not  make  jumps,"  it  follows 
that  every  break  which  we  see  in  the  continuity  is  not 
a  break  in  reality,  but  simply  in  our  knowledge  of 
history.  Many  breaks  which  formerly  existed  in  our 
knowledge  have  disappeared  with  advancing  dis- 
covery. It  is  natural,  then,  to  believe  that  the 
present  chasm  between  life  and  non-life  was,  at  the 
beginning  of  the  world  no  chasm,  but  filled  with  lost 
stages  which  can  never  be  recovered.  Speculations 
as  to  the  nature  of  these  lost  stages  have,  therefore, 
some  meaning  in  the  light  of  the  law  of  continuity. 
Scientists  do  not  look  upon  any  of  them  as  neces- 
sarily or  even  probably  true.  They  do  not  consider 
that  we  have  sufficient  knowledge  to  say  anything 
definite  upon  the  subject.  But  science  does  look  upon 
these  speculations  as  indicating  that  the  problem  of 
the  origin  of  life  is  not  an  insolvable  one.  Scientists 
take  them  for  what  they  are — pure  speculations,  but 
think  that  they  tell  us  that  the  break  at  the  beginning 
of  life  is  one  of  ignorance  and  not  one  of  fact. 

It  is  thus  only  the  supposed  existence  of  a  philo- 
sophical necessity  which  has  created  a  demand  for 
some  theory  of  a  natural  origin  of  life,  and  called 
into  existence  the  various  speculations  on  the  subject. 
The  conclusion  has  been  reached  that  the  general 
advance  of  thought  and  investigation  has  practically 
established  the  truth  of  the  law  of  continuity.  This 
law,  so  thoroughly  believed  in  by  modern  science, 
demands  the  destruction  of  the  chasm  between  the 
living  and  the  non-living.  Science  has,  therefore,  set 
to  work  to  destroy  it.  It  has  shown  that  the  chasm 


THE   ORIGIN  OF  LIFE.  49 

is  not  so  great  as  was  once  thought  ;  it  has  proved 
that  the  animal  body,  and  protoplasm  in  general,  is 
a  machine  making  use  of  the  chemical  energy  of  its 
foods ;  it  has  shown  that  growth  is  little  more  than 
chemical  change,  and  that  throughout  the  organic 
world  the  same  physical  and  chemical  forces  are  at 
play  as  in  the  inorganic  world,  only  under  more  com- 
plex conditions  ;  and  it  has  rendered  it  probable  that 
most  if  not  all  of  the  vital  properties  are  directly 
dependent  upon  and  explained  by  chemical  and 
physical  forces.  Science  has,  in  short,  proved  that 
living  processes  are  a  continuous  change  of  chemical 
and  physical  forces,  and  that  what  we  mean  by  life  is 
something  to  direct  this  play  of  force.  It  then 
assumes  that  this  something  is  to  be  accounted  for  as 
the  property  of  protoplasm  resulting  from  its  com- 
plex composition.  This  assumption  is  plainly  a  long 
step  in  the  region  of  hypothesis.  But  once  made,  it 
becomes  easy  to  posit  and  to  explain  by  speculation 
the  spontaneous  origin  of  life.  For,  indeed,  it  now 
follows  as  a  matter  of  necessity.  The  conclusion 
which  experiment  forces  upon  us,  that  spontaneous 
generation  does  not  occur  in  nature  to-day,  is  cast 
aside  as  irrelevant  to  the  more  fundamental  question. 
For  we  ought  not  to  expect,  even  if  life  originally 
did  appear  mechanically,  that  it  could  do  so  now, 
since  the  conditions  are  so  different.  Concerning  the 
first  origin  of  life,  science,  therefore,  knows  nothing, 
and  is  obliged  to  rest  satisfied  with  the  statement 
that  its  original  mechanical  origin  is  an  absolute 
necessity  of  thought.  "  To  hold  the  beginning  of 
life  as  an  arbitrary  creation  is  to  break  with  the 


50  THE  LIVING  WORLD. 

whole  theory  of  cognition,"  says  Zollner.  To  the 
scientist  who  is  convinced  of  the  universal  truth  of 
the  law  of  continuity,  therefore,  the  natural  origin  of 
life,  though  not  possible  now,  was  possible  and  did 
occur  in  early  times  under  conditions  about  which 
we  can  only  speculate.  Carbon  in  former  times 
certainly  did  crystallize  in  the  form  of  diamond, 
because  of  conditions  which  then  existed,  and  it  does 
not  do  so  now  because  of  the  absence  of  those 
conditions.  So,  we  are  told,  the  elements  carbon, 
oxygen,  hydrogen,  and  nitrogen,  did  in  former 
times  unite  together  to  form  protoplasm,  under 
conditions  which  then  existed,  but  have  long  since 
passed  away. 

But  this  may  seem  to  be  attacking  the  problem 
from  the  wrong  end.  The  law  of  continuity  is  the 
law  to  be  proved.  To  any  one  who  is  disposed  to 
question  the  far-reaching  significance  of  this  law  the 
matter  is  by  no  means  so  evident.  If  we  are  willing 
to  accept  the  existence  of  breaks,  few  or  many,  in 
the  history  of  the  universe,  we  may  well  place  one 
at  the  beginning  of  life.  The  break  exists  to-day,  at 
least,  and  no  amount  of  ingenious  speculation  is  suf- 
ficient to  cover  it.  Incapable  of  proof  or  disproof, 
demanded  by  no  bit  of  evidence,  and  if  we  do  not 
accept  the  law  of  continuity,  not  demanded  by  any 
philosophical  necessity,  these  speculations  of  science 
will  be  regarded  as  worthless.  They  are  laborious 
searches  after  something  which  does  not  exist.  But 
on  the  other  hand,  when  we  remember  how  persist- 
ently scientific  advance  is  leading  us  toward  a 
mechanical  conception  of  the  phenomena  of  nature, 


THE   ORIGIN  OF  LIFE.  51 

and  how  strongly  the  law  of  continuity  is  establish- 
ing itself  in  the  light  of  discovered  fact,  we  must 
admit  that  there  is  a  significance  in  speculations  on 
the  origin  of  life  which  is  deeper  than  at  first  sight 
appears. 

Protoplasm  Not  the  Simplest  Basis  of  Life. 

In  the  discussion  which  has  gone  before,  proto- 
plasm has  been  referred  to  as  if  it  were  a  definite 
chemical  compound,  though  one  of  great  complex- 
ity. It  has  seemed  to  many  that  the  first  step  in 
the  history  of  life  must  have  been  the  production  of 
a  protoplasmic  mass  like  some  of  our  present  low 
organisms  {Myxomycetes}.  But  nothing  is  more 
certain  than  that  protoplasm  is  not  a  definite  sub- 
stance, and  that  it  is  by  no  means  uniform  in  differ- 
ent organisms.  It  is  found  to  differ  widely  in  its 
physical  nature,  sometimes  being  almost  solid,  some- 
times very  liquid.  It  is  found  to  differ  more  or  less 
in  its  chemical  composition,  no  two  analyses  agreeing 
exactly,  and  when  we  take  its  functions  into  consid- 
eration, the  differences  between  different  specimens 
of  protoplasms  become  extreme.  Although  the 
amoeba  and  the  nerve  cell  of  the  human  brain  are 
both  composed  of  protoplasm,  how  wide  are  the 
differences  between  the  powers  they  possess.  Plant 
protoplasm  is  capable  of  making  use  of  the  energy 
of  sunlight,  while  animal  protoplasm  is  not.  When 
we  call  protoplasm  "  the  physical  basis  of  life,"  and 
try  to  prove  the  unity  of  the  organic  world  by  its 
universal  presence  in  all  living  things,  we  have  not 
by  any  means  reached  the  bottom  of  the  matter. 


52  THE  LIVING  WORLD. 

We  have,  it  is  true,  taken  one  step  farther  toward 
simplifying  the  life  problem  when  we  step  from 
the  study  of  the  cell  to  the  study  of  protoplasm,  but 
we  have  not  reduced  life  to  a  common  factor.  There 
is,  indeed,  no  such  thing  in  nature  as  protoplasm  in 
general,  but  only  particular  kinds  of  protoplasm. 
Protoplasm  can  produce  new  protoplasm,  but  not 
new  protoplasm  in  general,  only  new  protoplasm  just 
like  itself.  The  amoeba  can  make  new  amoeba  pro- 
toplasm and  no  other,  while  the  nerve  cell  is  capable 
of  giving  rise  to  new  nerve  protoplasm.  The  study 
of  the  variations  among  protoplasm  has  progressed 
until  we  are  slowly  ceasing  to  look  upon  it  as  a 
definite  substance.  The  microscope  investigation  of 
recent  years  has  shown  that  not  only  is  it  chemically 
complex,  but  it  can  no  longer  be  regarded  as  physi- 
cally homogeneous.  Several  distinct  chemical  com- 
pounds have  been  discovered  in  it.  Within  the 
simplest  protoplasm  has  been  found  an  extremely 
complex  structure.  There  is  a  dense  reticulum 
made  up  perhaps  of  hollow  threads,  containing 
minute  granules  and  a  liquid  matter  (Fig.  i).  The 
granules  are  united  in  various  ways  to  form  groups. 
To-day  we  may  almost  say  that  the  study  of  the 
structure  of  protoplasm  promises  to  become  as  com- 
plex a  subject  for  the  future  as  the  study  of  the 
animal  kingdom  on  a  large  scale  has  proved  to  be  in 
the  past.  Instead  of  looking  upon  protoplasm  as 
the  primitive  life  substance,  at  the  present  time, 
most  special  students  of  protoplasm  would  regard 
the  granules  as  more  likely  the  fundamental  part  and 
all  else  derived.  These  granules  are  excessively 


THE   ORIGIN   OF  LIFE. 


53 


minute,  requiring  the  highest  powers  of  the  micro- 
scope (four  thousand  diameters)  to  see  them  at  all, 
and  yet  they  are  now  looked  upon  as  having  in 
themselves  all  the  remarkable  powers  of  life.  We 
may  perhaps  even  say  that  wherever  there  is  a  single 


FIG.  i.     Protoplasm,  showing  its  reticulum,  very  highly  magnified. 

granule  there  is  life.  By  the  activities  of  the  gran- 
ules growth  takes  place,  by  their  differentiation  and 
union  and  by  the  products  of  their  growth  is  formed 
the  compound  vhich  we  have  called  protoplasm. 
By  their  aggregation  into  definite  groups  are  formed 
the  cells.  The  differences  between  cells  are  due  to 
the  differences  in  their  arrangements.  Protoplasm 


54  THE  LIVING  WORLD. 

of  itself  is  thus  ceasing  to  play  so  important  a  part 
in  biological  discussion  as  it  promised  to  do  ;  its 
place  as  a  primitive  life  substance  is  being  taken  by 
the  granules  with  their  various  products,  deuter- 
plasm,  microsomata,  etc.,  etc.,  and  in  scientific  litera- 
ture to-day  we  hear  less  and  less  of  protoplasm. 
Protoplasm  is  no  longer  the  physical  basis  of  life,  but 
if  indeed  scientists  should  to-day  recognize  anything 
junder  this  term,  it  would  probably  be  the  granules 
which  are  found  in  such  abundance  within  the  proto- 
plasm. And  yet  even  these  minute  bodies,  small  as 
they  are,  can  hardly  be  said  to  have  reached  their 
ultimate  analysis.  They  show  different  functions, 
in  different  cases,  and  cannot  be  looked  upon  as 
alike.  Whether  the  analysis  can  ever  be  carried 
farther,  of  course  we  cannot  say.  But  certain  it  is 
that  protoplasm  cannot  be  any  longer  strictly  re- 
garded as  the  physical  basis  of  life.  To-day  the 
only  universal  properties  of  protoplasm  seem  to  be 
that  it  is  reticulated  and  has  certain  powers  of  mo- 
tion, growth,  and  reproduction.  It  is  thus  seen  to 
be  always  a  secondary  rather  than  a  primitive  sub- 
stance, and  the  term  protoplasm  seems  to  be  an 
abstraction  from  the  idea  of  many  organisms,  just 
as  the  term  mankind  is  an  abstraction  from  many 
individual  men. 

There  are  therefore  many  steps  between  the  sim- 
plest protoplasm  yet  discovered  and  the  inorganic 
world,  or  even  between  it  and  the  simplest  form  of 
living  matter.  Hence  it  is  impossible  to  look  at  a  uni- 
formly diffused  mass  of  protoplasm  as  the  first  step  in 
the  life  history.  But  however  this  may  be,  it  will  be 
seen  that  it  does  not  materially  alter  the  significance 


THE    ORIGIN  OF  LIFE.  55 

of  the  scientific  speculation  of  the  present  chapter. 
In  showing  the  complex  nature  of  protoplasm,  the 
microscope  has  perhaps  given  a  little  more  founda- 
tion to  the  supposition  that  its  properties  are  due  to 
its  complex  composition,  physical  as  well  as  molecu- 
lar, but  has  not  of  course  removed  the  difficulty  of  a 
perpetually  automatic  machine,  acting  without  any 
directing  power  outside  of  itself,  and  has  made  its 
mechanical  origin  a  little  more  difficult  of  compre- 
hension. The  substitution  of  these  granules  for  pro- 
toplasm, moreover,  only  throws  the  question  of  the 
origin  of  life  a  little  farther  back  into  the  microscopic 
field,  but  does  not  alter  the  problem  as  to  the  forces 
which  brought  it  first  into  existence. 

Life  Appeared  in  the  Ocean. 

Although,  as  seen  above,  the  beginnings  of  life 
are  shrouded  in  darkness,  there  is  one  point  that  can 
be  stated  with  certainty.  There  is  no  d®ubt  that  the 
first  forms  of  life  appeared  in  the  ocean.  Protoplasm 
itself  is  largely  water,  and  in  the  water  it  must  have 
appeared.  At  the  early  periods,  when  life  appeared, 
there  was  probably  little  or  no  dry  land,  and  there 
could  have  been  no  bodies  of  fresh  water  to  serve  as 
the  starting-point  of  life.  Add  to  this  the  fact  that 
all  of  the  earliest  forms  of  life  found  as  fossils  are 
marine,  and  it  becomes  certain  that  the  ocean  must 
have  been  the  place  where  the  first  living  thing  made 
its  appearance. 

Summary. 

Life,  as  we  commonly  use  the  term,  is  certainly 
an  abstraction  from  the  properties  of  living  things. 


56  THE  LIVING  WORLD. 

Whether  in  its  ultimate  analysis  it  is  still  to  be  so 
regarded  is  an  open  question.  The  marvellous  pow- 
ers of  living  things,  and  the  great  mystery  surround- 
ing the  processes  of  growth  and  reproduction,  have 
caused  thinkers  to  believe  that  the  property  of  life 
was  something  more  than  a  simple  abstraction,  and 
to  regard  it  as. a  distinct  and  mysterious  force  resid- 
ing in  organisms,  capable  of  indefinite  expansion,  and 
having  its  origin  in  some  direct  creative  power. 
This  conception  has  been  retained  while  the  belief 
in  the  general  mechanism  of  nature  has  been  gaining 
universal  acceptance. 

This  conclusion,  that  of  the  vitalistic  school,  has 
for  some  time  been  subject  to  criticism.  The  mys- 
te'rious  activities  and  powers  of  living  things  have 
been  in  a  measure  explained  as  the  results  of  known 
forces  of  nature.  The  discovery  of  the  law  of  con- 
servation of  energy,  the  discovery  of  the  close  rela- 
tion of  chemistry  to  vital  processes,  the  manufacture 
of  organic  products  in  the  chemist's  laboratory,  and 
the  study  of  the  various  properties  of  the  complex 
compounds  of  carbon  have  all  united  in  proclaiming 
a  close  relation  between  the  laws  governing  the  liv- 
ing and  the  non-living  world.  It  was  inevitable  that 
the  conclusion  would  be  reached  that  what  we  call 
life  is  nothing  more  than  a  manifestation  of  ordinary 
forces  of  nature  under  special  conditions  furnished 
by  the  substance  protoplasm.  Such  a  position  is 
assumed  by  the  mechanical  theory  of  life. 

Even  when  this  position  is  taken,  the  fact  remains 
that  at  the  present  time  protoplasm  does  not  arise 
except  through  the  agency  of  other  protoplasm. 


THE   ORIGIN  OF  LIFE.  57 

We  are  thus  forced  to  ask  what  could  have  been  the 
origin  of  the  first  protoplasm  from  which  the  rest 
has  arisen.  After  all  the  discussion,  however,  we 
must  finally  admit  that  we  do  not  know  when  life 
first  arose  or  how,  nor  do  we  understand  the  causes 
which  brought  it  into  existence.  Many  secondary 
problems  have  been  and  are  being  solved,  but  the 
real  question  remains  as  yet  untouched,  except  by 
hypothesis  and  speculation.  Vital  processes  may 
all  be  shown  to  be  chemical  and  physical  processes, 
but  this  will  never  explain  why  they  are  carried  on 
automatically  in  protoplasm  and  here  alone ;  and 
granting,  if  we  are  inclined  to  do  so,  that  it  is  one  of 
the  physical  properties  of  protoplasm  to  direct  this 
play  of  force,  there  still  remains  the  fact  that  to-day 
protoplasm  can  only  come  from  other  protoplasm. 
Whence  came  the  first  protoplasm  ?  To  this  ques- 
tion science  offers  in  answer,  first :  the  law  of  con- 
tinuity, in  terms  of  which  the  original  spontaneous 
generation  of  life  from  the  non-living  is  a  necessity ; 
and,  second,  various  speculations,  which,  though  ac- 
knowledged to  be  entirely  unproved,  are  regarded 
as  showing  that  in  the  boundless  possibilities  of  the 
past,  spontaneous  generation  might  well  have  taken 
place,  provided  always  it  be  granted  that  life  is 
simply  the  result  of  complex  chemical  and  molecular 
composition. 

From  all  of  this  discussion  and  speculation  we  can 
isolate  two  facts:  I.  Life  arose  in  the  ocean.  2.  The 
first  form  of  life  was  the  simplest  possible  condition 
of  living  matter,  certainly  simpler  than  any  living 
organisms  with  which  we  are  acquainted  to-day,  and 


58  THE   LIVING   WORLD. 

very  likely  simpler  than  the  simplest  mass  of  diffused 
protoplasm.  Amid  the  confusing  and  unsatisfactory 
mass  of  fact  and  speculation  which  centres  around 
the  primal  origin  of  life  these  two  points  may  stand 
prominently  before  us  as  certain,  while  all  the  rest  is 
hypothesis,  with  various  degrees  of  probability. 


CHAPTER  III. 

THE   ORIGIN   OF   THE   ANIMAL   KINGDOM. 

WE  have  now  the  history  of  life  fairly  started. 
Living  matter  made  its  appearance  in  the  geological 
seas  of  the  far  distant  ages,  and  was  at  first  probably 
an  undifferentiated  mass  with  the  simplest  powers  of 
assimilation  and  growth.  Out  of  this  the  higher 
orders  of  life  were  to  arise,  and  we  have  now  to  study 
the  first  steps  toward  the  modern  world. 

Unicellular  Life. 

The  first  trace  of  anything  leading  toward  the 
modern  world,  of  which  we  find  evidence,  is  in  the 
formation  of  cells.  It  has  been  suggested  in  the  last 
chapter  that  there  first  existed  a  diffused  mass  of 
living  matter  without  differentiation  into  parts. 
Such  a  conclusion  has  not  been  proved,  and  if  such 
a  mass  did  exist,  it  probably  continued  only  a  short 
time.  It  soon  became  broken  up  into  small  masses 
which  were  more  or  less  independent  of  each  other. 
Our  reasons  for  this  conclusion  are  these :  The 
simplest  and  lowest  organisms  which  exist  to-day 
consist  of  just  such  little  independent  bits  of  proto- 
plasm which  we  call  cells  (unicellular  organisms). 

59 


60  THE  LIVING  WORLD. 

Secondly,  from  the  study  of  embryology  we  find 
that  all  animals  and  all  plants  begin  with  their  de- 
velopment as  single  cells,  and  since  embryology  re- 
peats past  history  this  of  course  points  to  a  former 
unicellular  condition  of  early  animals.  Here,  then, 
is  our  first  sure  starting-point.  For  a  diffused  mass 
of  protoplasm  at  the  beginning  of  life  we  have  only 
a  speculative  foundation,  but  for  a  unicellular  con- 
dition of  early  life  we  have  direct  evidence. 

Cells. 

Before  tracing  the  history  farther,  it  is  well  there- 
fore to  say  a  word  in  regard  to  the  meaning  of  the 
term  cell,  as  it  is  understood  to-day  by  naturalists. 
As  originally  used,  the  word  cell  was  a  good  de- 
scription of  the  object  so  named.  The  microscopic 
study  of  plants  showed  long  ago  that  they  are  in  all 
cases  composed  of  a  larger  or  smaller  number  of 
little  separate  compartments  or  little  boxes,  like  a 
mass  of  honeycomb.  Each  box  has  a  wall  and  con- 
tains a  mass  of  the  living  substance,  the  protoplasm. 
These  were  named  cells.  With  such  objects  as  a 
starting-point,  for  which  the  term  cell  was  a  good 
description,  the  word  has  been  extended  in  its  appli- 
cation until  it  has  come  to  be  applied  to  a  very 
different  set  of  objects.  It  was  soon  found  that  all 
plants  and  all  animals  were  composed  of  somewhat 
similar  independent  parts  and  the  name  cell  was 
naturally  extended  to  animals  as  well  as  plants. 
Inside  of  these  cells  there  was  subsequently  shown 
to  be  frequently  present  a  bit  of  dense  protoplasm 
quite  distinct  from  the  rest,  which  received  the  name 


THE    ORIGIN  OF   THE  ANIMAL   KINGDOM.      6 1 

of  nucleus.  In  many  cases,  however,  especially 
among  animals,  the  independent  bits  of  protoplasm 
were  found  to  have  no  wall  to  surround  them,  and 
many  cells  were  found  in  which  no  nucleus  could  be 
demonstrated.  Still  they  were  seen  plainly  to  be 


FIG.  2.    A  cell.—/'  Protoplasm.    N  Nucleus.    Ne  Nucleolus.     Civ  Cell  wall. 

independent  units  and  surely  to  deserve  the  name  of 
cells.  Thus  the  term  cell  came  to  mean  any  inde- 
pendent mass  of  protoplasm,  with  or  without  a 
nucleus,  with  or  without  a  cell  wall.  More  recently 
however,  it  has  been  quite  definitely  shown  that, 
though  the  cell  wall  is  not  always  present,  and  there- 
fore is  not  necessary  to  the  constitution  of  a  cell,  the 
nucleus  is  probably  always  present  in  active  cells, 
either  as  a  distinct  body  or  as  a  diffused  mass.  A 
cell  thus  becomes  an  independent  bit  of  protoplasm 
with  a  nucleus ;  or  better,  perhaps,  since  recent  study 
shows  the  fundamental  importance  of  the  nucleus,  a 
bit  of  nuclear  matter  with  surrounding  protoplasm. 


62  THE  LIVING  WORLD. 

Fig.  2  shows  a  cell  in  the  modern  sense.  All  of  the 
living  parts  of  animals  and  plants  are  composed  of  a 
mass  of  such  cells.  The  cells  however,  are  far  from 
being  all  alike.  Indeed,  they  differ  extremely  in 
shape  and  size  and  in  many  other  respects,  but  in  all 
cases  each  consists  of  a  bit  of  nuclear  matter  sur- 
rounded by  protoplasm,  and  each  is  thus  more  or 
less  independent  of  the  others.  The  larger  and 
higher  animals  are  composed  of  a  larger  number  of 
cells,  and  always  of  a  larger  variety  in  their  shape 
and  function. 

The  cell  has  thus  been  for  many  years  regarded  as 
the  unit  of  life,  and  the  life  of  the  individual  as  the 
sum  of  the  life  of  its  cells.  When  we  go  among  the 
lowest  and  simplest  of  animals,  we  find  one  large 
group  in  which  the  individual  consists  of  only  a 
single  cell.  These  are  the  unicellular  animals  {Pro- 
tozoa) and  plants  (Protophyta).  They  are  all  aquatic 
organisms,  all  small,  usually  microscopic,  all  of  course 
extremely  simple,  but  withal  extremely  abundant : 
they  are  found  in  the  ocean,  in  rivers,  brooks,  ponds, 
pools,  ditches,  swamps,  gutters,  puddles ;  in  short, 
wherever  there  is  any  water,  there  we  may  be  pretty 
sure  to  find  some  representatives  of  the  unicellular 
organisms.  Fig.  3  represents  a  few  types  of  these 
animals. 

It  was  a  great  simplification  of  our  knowledge  of 
animals  and  plants  when  it  was  discovered  that  all 
were  to  be  regarded  as  complexes  of  these  simple 
cells,  for  it  gave  a  unit  of  life.  The  unicellular  animals 
and  plants  were  naturally  regarded  as  the  simplest 
possible  organisms,  since  they  consisted  of  one  such 


THE   ORIGIN  OF   THE  ANIMAL   KINGDOM.      63 

unit.     It  was  therefore  extremely  significant  to  find 
that  in  their  embryology  all  animals  and  plants  alike 


FIG.  3.      Types   of   Protozoa.— I  Amoeba.       2  Actinospherum.      j  Paramecium. 
4  Vorticella  dividing.      5  Euglena.       6  Trichomonas.  '    7  Podophyra. 

began  life  as  a  single  cell,  the  egg  and  the  germ  cell 
of  the  plant  being  always  single  cells.  To  find  the 
cell  as  the  unit  of  life,  to  find  a  group  of  unicellular 


64  THE  LIVING  WORLD. 

organisms  occupying  the  lowest  position  in  the  scale 
of  nature,  and  to  find  all  animals  and  plants  begin- 
ning their  embryology  as  single  cells,  were  coinci- 
dences of  remarkable  interest.  There  can  be  only 
one  interpretation  of  this.  Since  embryology  is  an 
epitomized  account  of  past  history,  the  fact  that  all 
animals  and  plants  begin  life  as  single  cells,  of  course 
must  mean  that,  if  we  could  follow  back  the  history 
of  animals  and  plants,  we  should  in  each  case  finally 
come  to  some  unicellular  organism.  A  unicellular 
organism  was  therefore  a  common  starting-point  of 
all  animals  and  plants,  and  the  Protozoa  and  Proto- 
phyta  of  to-day  are  of  interest  as  being  close  repre- 
sentatives of  the  earliest  organisms  of  which  we  have 
any  suggestion  in  our  recorded  history. 

This  conclusion  of  a  unicellular  starting-point  of 
all  animals  and  plants  is  a  significant  result  of  bio- 
logical study.  At  the  same  time,  as  was  seen  in  the 
last  chapter,  more  recent  and  exhaustive  study  of 
cells  is  beginning  to  show  that  the  simple  cell  is  not 
itself  the  unit  of  life,  but  is  a  complex  body.  Pro- 
cesses are  found  to  take  place  inside  the  cell  which 
indicate  that  we  are  still  far  from  the  unit  of  life.  It 
is  seen  that  of  the  whole  cell  the  nucleus  is  really  the 
essential  part,  and  that  it  regulates  the  activities  of 
the  rest  (Fig.  2).  The  nucleus  itself  is  moreover  a 
complex.  No  fewer  than  five  different  chemical 
compounds  have  been  found  to  exist  in  it.  In  struc- 
ture also  we  find  various  fibres,  liquids,  and  granules, 
all  of  which  seem  to  undergo  definite  cycles  of  change 
in  the  activities  of  the  cell.  All  of  this  indicates  that 
we  are  still  far  from  the  unit  of  life  when  we  have 


THE   ORIGIN  OF    THE  ANIMAL   KINGDOM.      6$ 

reached  the  single  cell.  There  is,  perhaps,  as  broad  a 
series  of  phenomena  to  be  studied  between  the  single 
cell  and  the  real  unit  of  life,  as  those  which  we  have 
found  between  the  higher  animals  composed  of  many 
cells  and  the  single  cell.  Animals  have  all  been 
reduced  to  complexes  of  cells,  but  the  cell  bids  fair 
to  be  still  farther  reduced  to  a  complex  of  inconceiv- 
ably small  granules.  Be  that  as  it  may,  it  still  remains 
a  fact  that  in  the  history  of  life  we  have  not  yet 
discovered  any  direct  evidence  of  an  earlier  condition 
than  that  of  the  unicellular  organisms.  Embryology, 
which  is  our  first  source  of  evidence,  gives  us  a 
record  of  a  unicellular  condition,  but  of  nothing 
earlier  than  this.  The  first  step  that  we  can  take  in 
narrating  the  history  of  life  is,  therefore,  to  establish 
the  period  when  unicellular  animals  were  the  highest 
condition  of  organic  life  in  existence.  The  diffused 
protoplasmic  mother  of  life,  if  ever  such  existed,  had 
become  broken  into  independent  masses. 

Of  course,  nothing  can  be  determined  in  regard  to 
the  variety  of  form  of  these  early  organisms,  nor  can 
we  say  whether  they  were  all  alike  or  whether  they 
showed  variety  like  that  of  the  unicellular  organisms 
to-day.  Of  their  habits  we  know  nothing.  That 
each  organism  was  independent  and  capable  of  car- 
rying on  all  the  functions  of  life  within  itself  follows 
from  every  source  of  belief.  That  these  organisms 
multiplied  by  simple  division  is  certain  from  the  fol- 
lowing reasons :  Multiplication  by  division  is  a  uni- 
versal phenomenon  in  all  living  things  to-day.  In  its 
simplest  form  it  is  shown  by  most  of  the  existing 
unicellular  animals,  as  is.  illustrated  in  Fig.  4.  As 
5 


66  THE  LIVING  WORLD. 

there  shown,  multiplication  simply  consists  of  the 
division  of  the  original  cell  into  parts,  each  of  which 
is  like  the  other,  and  each  of  which  is  henceforth 
independent  of  the  others.  Now  such  multiplica- 
tion is  universal  in  the  unicellular  organisms ;  such 
a  multiplication  is  nearly  universal  among  the  cells 


FIG.  4.    Amoeba  in  the  act  of  dividing. — The  nucleus,  N,  has  already  separated 
in  two  parts. 

of  the  bodies  of  higher  animals  ;  such  a  multiplication 
is  the  universal  method  by  which  the  single-celled 
ovum  begins  to  grow  and  develop  into  the  many- 
celled  adult.  From  all  of  this  there  can  be  no  room 
for  doubt  that  such  a  method  of  multiplication  was 
possessed  by  the  earliest  unicellular  animals,  which 
we  must  assume  lived  at  the  very  beginning  of  the 
history  of  life  in  early  ages. 

* 

The  history  of  the  living  world  has  been  like  that 
of  a  branching  tree,  a  main  trunk  dividing  into 
branches,  and  these  in  turn  subdividing  until  they 


THE   ORIGIN  OF   THE  ANIMAL  KINGDOM.      67 

become  lost  in  thousands  of  small  twigs.  From  the 
time  of  branching  it  is  of  course  impossible  to  follow 
the  history  as  a  whole,  and  only  possible  to  follow 
that  of  the  various  branches.  At  the  point  which 
we  have  now  reached  we  have  come  to  the  end  of 
the  trunk,  and  we  find  the  first  division  into  two  large 
branches,  animals  and  plants,  as  mentioned  in  the 
preceding  chapter.  The  separation  between  these 
two  kingdoms  seems  to  have  occurred  even  while 
the  living  world  consisted  of  unicellular  organisms. 
From  this  point  we  must  follow  the  animals  and 
plants  separately.  We  shall  first  take  up  the  his- 
tory of  animals,  leaving  that  of  plants  for  later 
consideration. 

Early  History  of  the  Animal  Kingdom. —  The  Origin 
of  Multicellular  Animals. 

The  Protozoa  as  they  exist  to-day  may  be  fairly 
supposed  to  illustrate  the  early  unicellular  animals 
of  pre-historic  times.  The  next  stage  in  the  history 
of  life  was  the  appearance  of  the  multicellular  organ- 
isms. It  is  again  the  study  of  the  unicellular  ani- 
mals to-day  which  gives  us  suggestion  as  to  how 
this  change  arose.  Among  most  of  the  unicellular 
organisms,  when  a  cell  divides,  the  parts  completely 
separate  from  each  other  and  live  subsequently  inde- 
pendent lives.  Among  a  few  living  forms,  however, 
the  parts  do  not  completely  separate  from  each  other, 
but  remain  together  in  some  sort  of  connection  and 
are  more  or  less  dependent  upon  each  other.  Fig.  5 
shows  such  a  group  of  cells,  the  members  of  which 
have  arisen  from  the  division  of  a  single  one,  and  which 


68 


THE  LIVING  WORLD. 


have   not  completely  separated.     There  is  a  small 
amount  of  dependence  of  the  cells  upon  each  other ; 


FlG.  5.   A  Colony  of  Protozoa.— Each  cell  is  independent,     a  and  b  show  manner 
of  division  to  form  colonies. 

they  share  each  other's  food,  and  all  seem  to  be  con- 
nected by  some  physiological  bond.  Still,  each  is  in 
itself  complete  individual,  having  a  complete  system  of 


THE    ORIGIN  OF   THE  ANIMAL  KINGDOM.      69 

organs,  and  each  can  live  by  itself  perfectly  well  if 
separated  from  the  others.  Such  a  group  does  not 
form  a  multicellular  animal,  since  the  cells  are  each 
complete  individuals.  It  is  rather  a  colony  of  uni- 
cellular animals.  Nevertheless,  such  a  colony  is  the 
first  step  toward  the  production  of  the  multicellular 
organisms.  From  embryological  evidence  also  there 
can  be  little  doubt  that  such  was  the  method  by 
which  the  multicellular  animals  and  plants  began  to 
develop  in  the  early  history  of  the  world.  We  have 
no  means  of  determining  to  what  extent  this  forma- 
tion of  such  colonies  of  independent  animals  occurred. 
It  was  indeed  only  a  stepping-stone  toward  the  next 
stage  in  the  history  of  living  things,  a  stage  of  much 
more  importance,  viz.,  the  formation  of  the  first  true 
multicellular  organism. 

To  form  a  multicellular  organism  it  is  not  sufficient 
that  there  should  simply  be  a  large  number  of  cells 
attached  together.  This  occurs  in  many  of  the  ani- 
mals that  are  regarded  as  unicellular.  In  order  that 
there  should  be  a  true  multicellular  animal,  a  meta- 
zoan  in  distinction  from  the  protozoan,  it  is  necessary 
that  there  should  be  a  certain  amount  of  division  of 
labor  among  the  cells.  So  long  as  each  cell  carries  on 
all  the  functions  of  life  in  itself  the  aggregation  of 
cells  forms  simply  a  colony,  but  when  the  different 
cells  in  such  a  group  begin  to  assume  different  func- 
tions— e.g.,  some  of  them  capturing  food  and  others 
digesting  it, — then  the  different  cells  become  strictly 
dependent  upon  each  other,  and  there  arises  a  true 
multicellular  animal.  A  multicellular  animal  is  one 
in  which  there  is  an  aggregation  of  cells,  each  per- 


70  THE  LIVING  WORLD. 

forming  only  a  part  of  the  functions  of  life,  and  thus 
each  dependent  upon  the  others.  The  whole  forms 
a  unit.  In  such  a  community  it  is  no  longer  possible 
for  a  single  cell  to  be  separated  from  the  rest  and 
still  continue  its  life,  for  such  a  cell  would  be  able  to 
perform  only  those  duties  for  which  it  was  adapted, 
and  it  would  therefore  soon  die. 

The  Gastrcza,  the  Common   Trunk  of  the  Animal 
Kingdom. 

That  such  a  multicellular  community  arose  in  the 
early  history  of  life  from  the  unicellular  forms,  is  of 
course  evident,  and  that  it  arose  by  cell  division 
which  did  not  become  complete  enough  to  separate 
the  individual  cells  from  each  other,  is  also  almost 
certain.  Exactly  how  a  division  of  labor  first  arose, 
or  what  that  first  division  may  have  been,  it  is  perhaps 
impossible  to  say.  Indeed,  it  is  not  improbable  that 
in  the  early  history  of  life  there  may  have  arisen 
many  different  types  of  division  of  labor  giving  rise 
to  different  kinds  of  true  multicellular  communities. 
But  whatever  may  have  been  the  early  varieties  of 
such  differentiation,  the  very  important  fact  is  true 
that  only  one  of  these  early  types  of  multicellular 
animals  perpetuated  itself  so  as  to  affect  the  subse- 
quent development  of  animals.  The  study  of  embry- 
ology shows  that  all  of  the  subsequent  types  of  the 
animal  kingdom  arose  from  one  definite  type  of  early 
differentiation.  In  other  words,  of  all  the  types  of 
cell  aggregates  or  communities  which  may  have  been 
produced  in  the  early  history  of  life  only  one  of  them 
proved  itself  of  value  enough  to  take  a  definite  place 


THE   ORIGIN-  OF   THE  ANIMAL  KINGDOM.      7 1 

in  the  history  of  animals,  and  from  this  one  all  other 
types  of  animals  have  developed. 

The  early  type  of  animal  thus  referred  to  as  of  so 
much  importance  in  the  history  of  living  things  is 
one  of  which  we  have  abundant  evidence  in  embry- 
ology, though  perhaps  the  type  does  not  exist  to-day 
as  a  distinct  animal.  Since  embryology  is  the  only 


FIG.  6.    A  Typical  Gastrula.— The  shaded  cells,  en,  are  those  connected  with 
the  digestion  of  food,    en  Endoderm.    ec  Ectoderm. 

evidence  of  this  stage  in  the  history  of  life,  our 
knowledge  of  this  early  form  is,  for  reasons  shown, 
confined  to  its  fundamental  structure  with  practically 
no  sure  knowledge  of  its  details.  The  type  in  ques- 
tion has  been  named  the  Gastraea  by  Haeckel,  who 
first  clearly  saw  that  embryology  taught  that  such  a 
common  ancestor  formerly  existed.  The  evidence  for 
this  conclusion  is  as  follows :  It  has  been  found  by  a 


72  THE  LIVING  WORLD. 

long-continued  study  of  embryology  that  all  multi- 
cellular  animals  pass  through  a  stage  which  embryol- 
ogists  have  called  a  gastrula.  A  typical  gastrula  is 
shown  in  Fig.  6.  It  is  seen  to  consist  of  a  cup  made 
of  two  layers  of  cells,  one  of  which  forms  the  outside 
of  the  cup  and  the  other  the  inside.  When  the  two 
layers  are  thus  formed  they  assume  different  func- 
tions ;  the  outer  layer  of  cells  becomes  especially 
connected  with  the  powers  of  motion,  sensation,  and 
the  other  functions  directly  connected  with  the  outer 
world.  The  inner  cells  being  removed  from  direct 
contact  with  the  exterior,  and  being  in  a  measure 
protected,  take  for  their  function  the  duty  of  the 
digestion  of  food  which  is  supplied  to  them  by  the 
outer  layer  of  cells,  the  opening  serving  for  both 
mouth  and  anus.  Now  all  animals  show  evidence  of 
having  passed  through  some  such  stage  as  this  gas- 
trula, and  the  interpretation  of  the  fact  can  mean  but 
one  thing.  If  embryology  repeats  past  history  this 
must  mean  that  near  the  beginning  of  the  develop- 
ment of  the  aru'mal  kingdom,  there  was  a  common 
ancestor  which  corresponded  in  its  fundamental  feat- 
ures with  this  gastrula  stage,  and  further,  that  all 
animals  which  at  the  present  time  show  traces  of  this 
stage  in  their  development  have  descended  from  that 
common  ancestor.  To  the  common  ancestral  stage 
thus  indicated,  Haeckel  gave  the  name  Gastraea,  a 
name  which  is  seen  to  correspond  to  the  embryologi- 
cal  stage,  the  gastrula.  This  name  we  will  retain  in 
the  following  discussion,  although  it  is  very  probable 
that  the  Gastraea,  as  Haeckel  understood  it,  never 
existed.  The  idea  as  Haeckel  conceived  it  has 


THE   ORIGIN  OF    THE  ANIMAL   KINGDOM.      73 

undergone  some  considerable  modification  within 
recent  years,  but  these  modifications  do  not  alter  the 
wide  significance  of  the  common  appearance  of  this 
stage  in  the  embryology  of  animals.  While  then  the 
Gastraea  as  originally  conceived  may  be  modified  in 
future  years,  there  is  no  probability  that  future  study 
will  make  any  less  significant  the  early  ancestor 
whose  fundamental  structure  was  essentially  that  of 
the  gastrula. 

We  may  start  the  history  of  multicellular  animals 
then  by  supposing  that  the  unicellular  animals  first 
became  aggregates  of  cells,  and  then  that  those  cells 
which  were  situated  on  the  outside  of  the  mass 
acquired  special  functions  connected  with  the  rela- 
tion of  the  animal  to  the  external  world,  while 
the  cells  which  were  in  the  interior  of  the  mass  took 
for  their  duty  the  digestion  of  food  which  was 
passed  to  them  from  the  exterior.  In  other  words, 
there  was  a  division  of  labor  between  the  cells  of  the 
animal,  and  when  this  had  appeared,  there  arose  the 
first  true  multicellular  animal.  While  we  have  no 
fossil  evidence  of  the  form  of  the  first  multicellular 
animal  which  appeared  in  the  world,  the  evidence 
derived  from  embryology  tells  us  pretty  surely  that 
it  must  have  been  one  which  could  be  described  as  a 
two-layered  sac,  probably  open  at  one  end,  for  the 
injection  of  food  and  the  ejection  of  waste,  whose 
inner  lining  served  for  digestion,  and  whose  outer 
covering  possessed  motor  and  nervous  functions. 

As  a  further  confirmation  of  the  conclusion  thus 
reached  from  embryology,  it  is  highly  important  to 
notice  that  there  is  still  in  existence,  among  the 


74 


THE   LIVING  WORLD. 


lower  marine  and  fresh-water  animals,  a  type  scarcely 
more  than  such  a  two-layered  sac.  The  Coelenterata 
is  perhaps  in  some  respects  the  lowest  group  of 
multicellular  animals,  and  among  this  group  some 
orders  have  a  structure  which  is  almost  exactly  that 


FIG.  7.     Diagram  of  the  structure  of  Hydra. — It  differs   from   the  gastrula  of 
Fig.  6  only  in  having  the  body  around  the  mouth  expanded  into  tentacles,  t. 

above  described  as  characterizing  the  first  multi- 
cellular  animal.  Hydra  (Fig.  7)  is  readily  seen  to 
be  simply  a  two-layered  sac,  with  an  opening  at  one 
end.  The  functions  of  the  two  layers  of  Hydra  are 


THE   ORIGIN  OF   THE  ANIMAL   KINGDOM.      75 

the  same  as  those  supposed  to  have  been  in  the 
early  primitive  animal.  Among  other  coelenterates 
this  simple  type  is  more  or  less  modified,  but  it  can 
always  be  seen  to  have  the  same  fundamental  plan. 
Since,  then,  the  lowest  existing  group  of  multicellu- 
lar  animals  is  essentially  the  same  in  structure  as  the 
Gastraea,  to  which  embryology  teaches  us  to  refer  all 
animals,  we  must  conclude  that  the  former  existence 
of  the  Gastraea,  as  the  starting-point  of  multicellular 
animals,  is  one  of  the  best  attested  facts  of  biological 
science.  It  is  a  conclusion  that  can  be  gainsaid  only 
by  denying  completely  the  value  of  embryological 
and  anatomical  evidence,  and  this  would  of  course 
be  to  deny  the  cogency  of  biological  science  com- 
pletely. 

The  first  step  in  the  history  of  life  toward  the 
development  of  the  higher  animals  may  then  be 
briefly  summarized  as  follows  :  Some  original  uni- 
cellular animals,  in  the  course  of  their  repeated 
divisions,  failed  to  separate  completely  into  single 
cells  after  dividing,  but  the  cells  thus  produced 
remained  attached  to  each  other.  At  first  the  cells 
were  all  alike,  and  had  similar  functions,  but  after  a 
time  the  cells  on  the  outside  and  those  on  the  inside 
of  the  mass  began  to  perform  different  duties. 
Those  on  the  inside  could  no  longer  use  any  powers 
of  motion,  and  the  possession  of  sensitive  functions 
would  be  useless,  since  they  had  no  direct  relations 
with  an  external  world  to  excite  the  sensations.  It 
is  a  law  of  nature  that  any  power  which  is  not  used 
begins  to  degenerate,  and  therefore  the  internal 
cells  soon  lost  their  motor  and  sensory  functions. 


76  THE  LIVING  WORLD. 

Since  they  were  protected  from  internal  injury,  they 
could,  on  the  other  hand,  better  perform  certain 
functions  connected  with  the  preparing  of  the  food, 
provided  that  they  could  succeed  in  getting  hold  of 
the  food  itself.  The  outer  layer  of  cells,  coming  into 
direct  contact  with  the  world,  retained  all  of  the 
powers  which  enabled  it  to  be  stimulated  by  that 
world,  and  soon  learned  to  pass  food  to  the  inner 
layer  of  cells  for  digestion. 

We  are  still  in  the  dark  as  to  the  exact  manner  in 
which  the  special  form  known  as  the  Gastraea  arose. 
According  to  some  embryologists,  the  cells  first 
formed  a  hollow  sphere,  and  then  one  side  of  it  was 
infolded,  as  one  would  push  in  the  side  of  a  hollow 
rubber  ball.  According  to  others,  the  mass  of  cells 
was  solid,  the  outer  ones  soon  becoming  different 
from  the  inner  ones,  and  later  a  cavity  appeared  in 
the  middle,  which  broke  through  to  the  exterior  at 
one  end,  and  this  opening  formed  the  mouth.  Ac- 
cording to  still  another  view,  the  cells  at  first  formed 
a  flat  mass  of  two  layers  of  cells,  and  this  by  folding 
up  into  a  cup,  or  going  through  other  modifications, 
became  the  two-layered  sac  which  forms  the  first 
distinct  stage  in  the  development  of  the  multicellular 
animals.  According  to  others  still,  and  this  is  the 
most  recent  view,  a  hollow  sphere  was  formed,  and 
thus  cells  from  the  shell  migrated  into  the  interior, 
one  by  one,  to  form  the  internal  layer.  The  mouth 
opening  was  developed  later.  But  whatever  differ- 
ence there  may  be  in  our  ideas  as  to  the  details  of 
this  formation,  no  biologist  questions  the  fact  that 
very  early  in  the  history  of  the  living  v/orld  there 


THE   ORIGIN  OF   THE  ANIMAL   KINGDOM.      77 

was  developed  an  animal  with  an  outer  and  an  inner 
layer  of  cells,  and  a  mouth  opening,  and  that  this 
form  has  been  the  starting-point  of  most,  if  not  all, 
of  the  subsequent  multicellular  animals.  We  shall 
call  it  the  Gastraea,  although  it  may  not  have  been 
exactly  the  same  sort  of  animal  as  that  to  which 
this  name  was  originally  applied. 

Divergence  of  Types. 

The  next  step  in  our  history  of  animals  was  one  of 
great  importance.  The  Gastraea  became  moulded  into 
types  which  foreshadowed  the  animal  world  of  to- 
day. Of  the  modifications  of  the  Gastraea  by  which  it 
became  changed  into  the  various  types  of  higher 
animals,  embryology  and  anatomy  alone  give  us  evi- 
dence, and  even  here  the  evidence  is  clear  only  in  a 
few  cases.  There  seems  to  be  conclusive  proof  that 
the  Gastraea  was  the  last  stage  which  was  shared  in 
common  by  the  different  groups  of  animals,  and 
possibly  some  groups  branched  off  even  earlier  than 
this  typical  Gastraea  stage.  From  this  point  cer- 
tainly a  divergence  took  place  which  soon  resulted 
in  the  formation  of  a  number  of  animal  types  which 
we  now  recognize  as  the  sub-kingdoms  of  animals. 
The  details  of  this  divergence  embryologists  have 
not  yet  fully  mastered.  The  simplest  case  seemed 
to  be  along  the  one  line  which  gave  rise  to  the  cce- 
lenterates.  In  this  line  of  descent  the  original  Gas- 
traea attached  itself  by  the  end  opposite  the  mouth, 
and  by  then  undergoing  various  small  changes  in 
form,  such  as  production  of  tentacles  and  elongation 
of  the  body,  produced  the  group  of  animals  of  which 


^8  THE   LIVING  WORLD. 

Fig.  8,  g,  is  an  example,  the  group  of  hydroids 
including  corals,  jelly-fishes  and  sea  anemones.  In 
nearly  all  other  lines  of  descent  from  the  Gastraea 
there  was  soon  acquired  a  new  fundamental  feature. 
The  mouth,  which  originally  served  for  the  entrance 
of  food  and  the  excretion  of  waste  matter  as  well, 
was  replaced  by  a  pair  of  openings,  one  serving  for 
each  function  (Fig.  8,  b).  Just  how  this  took  place 
is  perhaps  still  a  little  problematical.  The  proba- 
bility seems  to  be  that  the  one  opening  of  the  Gastraea 
elongated  into  a  slit  and  then  closed  in  its  middle. 
The  two  ends  of  this  elongated  slit  remained  open, 
however,  one  for  the  entrance  of  food,  and  the  other 
for  the  exit  of  refuse.  With  the  subsequent  elonga- 
tion of  the  body  these  two  openings  became  widely 
separated  from  each  other.  After  this  further 
modifications  of  the  body  arose.  Developing  a  shell 
on  its  back  (Fig.  8,  e),  it  started  along  a  line  which 
has  produced  the  type  which  we  have  called  mol- 
lusks  (snails,  oysters).  Along  one  line  of  descent 
this  shell  assumed  a  spiral  twist,  giving  rise  to  the 
snails  (Fig.  8,  /).  In  another  the  shell  became 
divided  into  two  pieces,  one  on  either  side,  giving 
rise  to  the  clams,  oysters,  etc.  ( ' Lamellibranchiata). 
In  another  line  it  became  elongated  and  divided  into 
segments,  and  this  gave  rise  to  the  large  type  of 
animals  known  as  segmented  animals  (Fig.  8,  d\ 
(segmented  worms,  Crustacea,  Insectd).  An  elonga- 
tion and  segmentation  and  subsequent  modification 
in  many  important  particulars,  the  chief  of  which 
was  the  development  of  an  internal  skeleton,  pro- 
duced the  type  of  Vertebrata.  The  details  of  these 


THE    ORIGIN  OF    THE  ANIMAL   KINGDOM.       7 


79 


changes,  however,  need  not  delay  us.  They  are 
technical  in  nature  and  could  only  be  understood  by 
one  acquainted  with  comparative  anatomy  and  em- 


FIG.  8.  Diagrams  illustrating  the  origin  of  the  Coelentera  Mollusca,  and 
Annelida  from  the  gastrula. — To  produce  the  Ccelentera  a  becomes  directly 
modified  into^-.  To  produce  the  Annelida  the  line  of  development  is«,  £,<:,  d. 
To  produce  the  Mollusca  it  is  a,  b,  e^f.  In/" the  twist  that  appears  in  the  shell 
twists  the  digestive  canal,  m,  mouth ;  «,  anus  ;  j/z,  shell ;  wy,  cells  which  are 
to  develop  into  muscles. 

bryology.  Nor,  indeed,  must  it  be  understood  that 
the  embryologist  can  follow  them  in  all  cases  even  to 
his  own  satisfaction,  nor  that  all  embryologists  agree. 
Various  methods  of  accounting  for  the  origin  of  the 


UJI7H.RSITT; 


-«W« 


8O  THE   LIVING  WORLD. 

vertebrates  are  advanced.  The  relations  of  the  star 
fishes  (Echinoderms))  the  group  of  low  worms 
(Vcrmes),  and  some  other  types  still  prove  a  puzzle 
to  him,  and  it  will  doubtless  require  much  investiga- 
tion still  before  the  embryological  history  is  fully 
elucidated. 

The  history  of  the  development  of  the  primitive 
Gastraea  into  our  modern  types  is  a  matter  of  specu- 
lative interest  to  the  specialist,  but  to  the  general 
reader  is  too  complex  to  make  it  worth  while  to 
dwell  upon  it.  There  is,  however,  concealed  in  this 
subject  a  general  principle  of  development  of  the 
most  extreme  importance.  From  the  history  of  this 
Gastraea  we  learn  that  the  divergence  of  the  great 
types  of  animals  must  have  occurred  early  in  the 
history  of  animals,  and  that  no  new  great  types  have 
appeared  except  in  the  early  history.  The  reason  for 
this  is  easily  seen.  The  differences  which  separate 
the  great  types  of  animals  from  each  other  are  in 
points  of  fundamental  structure  and  plan.  Such  dif- 
ferences could  have  appeared  only  in  the  descend- 
ants of  some  early  form  in  which  no  special  type  had 
appeared.  After  the  line  of  descendants  had  assumed 
a  definite  type,  it  is  not  likely  that  they  would  ever 
afterwards  have  changed  their  type  though  many 
changes  in  minor  details  may  have  occurred.  In- 
deed, all  evidence  shows  us  that  the  types,  after 
they  have  once  become  fully  established,  have  not 
changed  their  plan  but  have  remained  constant.  A 
tree  when  it  starts  from  the  ground  as  a  seedling 
soon  gives  rise  to  several  branches.  Now  this  early 
branching  which  takes  place  in  a  few  days  after  the 


THE   ORIGIN  OF   THE  ANIMAL   KINGDOM.      8 1 

seed  springs  from  the  ground,  really  determines  the 
subsequent  shape  of  the  tree.  No  matter  if  the  tree 
lives  to  be  a  hundred  years  old  it  will  always  be  pos- 
sible to  see  in  its  giant  limbs  the  early  branching  of 
the  seedling.  These  early  branches  become  larger 
ones,  and  they  in  turn  give  rise  to  smaller  branches 
and  twigs,  but  after  the  first  few  weeks'  growth  it  is 
impossible  for  the  plant  to  produce  any  more  pri- 
mary branches.  So  in  a  modified  way  it  seems  true 
in  the  animal  kingdom.  Very  early  in  the  history  of 
animals,  the  Gastraea  trunk  branched  into  several 
primary  divisions,  and  these  have  continued  to  the 
present  time.  They  have  grown  larger,  they  have 
produced  many  subdivisions,  but  the  animal  king- 
dom did  not,  after  the  early  periods  of  its  history, 
produce  new  primary  divisions.  In  other  words,  no 
new  sub-kingdoms  have  arisen  since  the  earliest 
periods  of  the  development.  (For  Vertebrata  see 
Chapter  IV.) 

It  is  possible  to  reach  this  same  conclusion  from 
another  standpoint.  The  development  of  the  ani- 
mal kingdom  has  been  in  all  cases  from  undifferen- 
tiated  to  differentiated.  Organs  originally  all  alike 
and  adapted  to  simple  functions,  have  become  dif- 
ferent from  each  other  and  adapted  to  more  com- 
plex functions.  For  instance,  the  mass  of  muscles 
which  in  the  fish's  body  are  adapted  only  to  the 
flexion  of  the  body  from  side  to  side,  and  the  simple 
motions  of  its  fins,  become  in  the  more  developed 
vertebrates,  like  man,  divided  into  several  hundred 
separate  muscles,  each  with  its  own  function,  all  result- 
ing in  the  complicated  powers  of  motion  shown  by 


82  THE  LIVING    WORLD. 

his  body.  The  development  of  animals  has  always 
been  thus  a  differentiation.  But  we  must  bear  in 
mind  that  only  the  undifferentiated  can  become 
differentiated,  and  it  is  plain  that  after  an  animal 
has  once  become  differentiated  in  any  direction  the 
possibilities  for  further  differentiation  become  imme- 
diately limited.  Let  us  notice  a  single  familiar  ex- 
ample in  illustration.  At  one  time  in  the  history  of 
the  horse  family  the  possession  of  five  toes  on  each 
foot  was  a  common  character.  From  a  five-toed 
condition  a  large  number  of  lines  of  descent  were 
possible.  But  the  actual  animals  entered  upon  a 
line  of  progress  which  resulted  in  the  successive  loss 
of  four  of  their  toes.  Now  with  each  step  in  this 
progression  the  possibilities  for  further  development 
became  limited.  The  animal  with  four  toes  \vas 
forever  debarred  from  all  lines  of  progression  that 
required  five,  and  the  horse  of  to-day,  having  only 
one  toe,  has  practically  ended  the  possibilities  of  de- 
velopment in  this  direction.  This  one  toe  he  must 
of  course  retain  ;  and  since  it  is  a  law  of  biology  that 
an  organ  once  lost  is  never  redeveloped,  any  further 
differentiation  of  the  toe  of  the  horse  is  impossible. 
Now  the  same  principle  is  true  everywhere.  As 
soon  as  the  first  step  in  any  line  of  differentiation  is 
taken,  the  possibilities  for  further  development  be- 
come immediately  limited. 

The  most  undifferentiated  of  all  types  of  multi- 
cellular  animals  is  the  Gastraea.  In  this  simple 
structure  there  are  possibilities  of  an  immense 
amount  of  modification,  simply  because  it  has 
developed  no  structure  of  its  own.  But  just  as 


THE   ORIGIN  OF   THE  ANIMAL  KINGDOM.      83 

soon  as  the  descendants  of  this  simple  type  become 
modified  in  any  direction,  the  future  development  is 
immediately  limited  to  the  type  thus  produced. 
When  some  of  the  early  animals  modified  their 
digestive  organs  so  that  there  were  two  openings  to 
the  digestive  tract  instead  of  one,  from  that  time 
their  descendants  were  required  to  conform  to  this 
type.  When  some  of  them  developed  an  elongated 
segmented  body,  it  was  no  longer  possible  for  them 
to  return  to  the  Gastraea  stage,  and  start  in  a  new 
direction.  Advance  in  structure  comes  from  as- 
sumption of  different  functions  on  the  parts  of 
organs  originally  alike,  and  as  the  parts  of  the  body 
acquire  a  wider  and  more  varied  development,  the 
possibility  of  further  differentiation  is  more  and 
more  restricted.  The  further  the  development  of 
animals  progresses,  therefore,  the  less  will  be  the 
modifications  of  structure  that  take  place,  and  the 
more  must  development  be  confined  to  the  elabora- 
tion of  details.  This  fact  we  find  constantly  illus- 
trated throughout  the  history  of  animals,  and  it  will 
be  frequently  referred  to  hereafter. 

We  can  thus  understand  why  the  sub-kingdoms 
which  arose  early  in  the  life  of  animals  could  con- 
tinue to  advance  and  elaborate  each  its  own  type  by 
the  production  of  many  minor  branches,  but  why 
none  of  them  could  give  rise  to  new  types  which 
would  present  as  great  differences  as  the  sub-king- 
doms first  appearing.  If  the  Gastrsea  ancestor  had 
remained  in  existence  to  serve  in  future  ages  as  the 
starting-point  of  new  lines  of  differentiation,  then 
the  production  of  great  divisions  would  seemingly 


84  THE  LIVING  WORLD. 

have  been  indefinitely  continued.  But  such  does 
not  appear  to  have  been  the  case.  In  other  words, 
the  evidence  of  nature  seems  to  indicate  that  the 
different  sub-kingdoms  did  not  develop  from  each 
other,  but  all  (?)  diverged  early  in  the  history  of  the 
world  from  a  simple  undifferentiated  animal  which 
bore  some  resemblance  to  the  hypothetical  Gastraea. 
This  conclusion  is  one  of  no  little  significance,  for  it 
indicates  that  the  origin  of  the  large  types  of  ani- 
mals was  a  matter  much  more  rapid  than  their  sub- 
sequent elaboration.  At  the  origin  of  the  multi- 
cellular  animals,  a  comparatively  short  time  probably 
produced  divergence  in  the  type  which  gave  rise  to 
the  great  sub-kingdoms,  while  the  millions  of  years 
that  succeeded  only  sufficed  to  elaborate  and  differ- 
entiate these  types.  This  understanding  gives  us  an 
explanation  of  the  interesting  fact  that  no  new 
types  have  appeared  within  the  geological  periods 
since  the  Silurian,  and  it  assists  very  much  in  under- 
standing what  seems  to  be  the  sudden  appearance 
of  life  in  the  oldest  fossiliferous  rocks. 

Summary. 

In  the  study  of  the  early  history  of  life  three 
points  of  importance  stand  prominently  forth. 

First :  Far  back  at  the  beginning  of  life  on  the 
globe,  there  was  a  period  during  which  the  unicellu- 
lar organisms  were  the  highest  that  existed.  Al- 
ready the  plant  life  and  the  animal  life  had  become 
separate.  We  do  not  understand,  however,  that 
this  unicellular  stage  was  the  beginning  of  the  his- 
tory of  life,  for  we  are  learning  that  there  is  a  vast 


THE    ORIGIN   OF    THE   ANIMAL   KINGDOM.      8$ 

field  lying  below  the  simple  cell :  but  none  of  our 
sources  of  evidence  give  us  as  yet  any  sure  traces 
of  this  earlier  history.  Of  the  unicellular  stage  of  the 
history  we  are  sure.  Without  being  able  to  deter- 
mine much  in  regard  to  them,  we  may  perhaps  look 
upon  the  unicellular  organisms  existing  to-day  as  a 
tolerably  correct  picture  of  those  of  early  times. 

Second  :  We  learn  to  picture  to  ourselves  these 
early  unicellular  forms  as  multiplying  by  division, 
and  we  see  among  them  many  instances  where  the 
multiplication  did  not  produce  complete  separation 
of  the  cells.  The  cells  remained  attached  to  each 
other  and  formed  colonies  of  unicellular  animals. 
Among  these  colonies  a  differentiation  of  the  cells 
and  of  function  began  to  make  its  appearance.  To 
what  extent  different  types  of  this  early  differentia- 
tion appeared,  we  do  not  know,  but  finally  there 
arose  one  which  has  proved  permanent.  This  was 
the  two-layered  cup  to  which  has  been  given  the 
name  of  Gastraea.  This  simple  type  with  its  mouth 
and  stomach,  proved  itself  strong  enough  to  battle 
for  life  with  nature,  and  it  probably  became  the 
starting-point  of  all  subsequent  animals.  The 
kingdom  of  plants  separated  itself  permanently 
from  the  animals,  and  can  no  longer  be  traced  with 
them. 

Third ;  The  Gastraea  was  the  last  point  of  com- 
mon origin  to  which  the  different  sub-kingdoms  can 
be  traced.  From  here  there  arose  a  divergence 
which  soon  produced  the  types  of  the  existing 
animal  world.  Moreover,  we-  have  seen  that  not 
only  is  it  impossible  to  trace  the  whole  animal  world 


86  THE  LIVING  WORLD. 

to  any  later  point  of  origin,  but  probably  most,  if 
not  all,  of  the  sub-kingdoms  can  be  independently 
traced  to  this  one  point,  indicating  that  this  was  the 
starting-point  of  all  the  animal  kingdoms.  It  is 
doubtful  whether  even  two  of  the  great  types  exist- 
ing to-day  have  had  a  common  point  of  origin  later 
than  the  Gastraea.  This  has  been  seen  to  be  due  to 
the  fact  that  the  great  types  are  separated  from  each 
other  by  differences  in  fundamental  plan  of  structure, 
and  such  fundamental  differences  could  only  have 
arisen  from  an  undifferentiated  form  which  had  as 
yet  developed  no  distinct  type  of  its  own. 


CHAPTER  IV. 

THE    RECORD    FROM    FOSSILS. 

WE  have  thus  far  traced  the  history  of  animals 
through  the  unicellular  stage  to  the  Gastraea  type, 
and  we  have  seen  that  this  Gastraea  probably  diverged 
rapidly,  by  various  modifications  in  the  shape  and 
structure  of  its  body,  into  different  sub-kingdoms. 
Thus  far  our  evidence  has  been  only  sufficient  to 
indicate  such  general  facts  with  no  details.  Embry- 
ology, with  the  assistance  of  comparative  anatomy, 
has  told  us  all  that  we  know  of  this  early  history  of 
living  things,  and  these  two  sources  of  evidence,  as 
already  pointed  out,  can  give  no  details.  Still  it  is 
plain  that  the  ground  on  which  we  are  treading  is 
much  more  sure  than  that  which  served  as  our  foot- 
ing, while  trying  to  discover  facts  in  regard  to  the 
primitive  origin  of  life.  While  our  knowledge  as 
to  the  origin  of  life  is  all  hypothesis,  and  we  are 
not  even  sure  of  general  facts,  our  knowledge  of  the 
divergence  of  the  great  types  from  the  Gastraea  is 
more  than  hypothesis,  and  of  the  general  facts  out- 
lined in  the  last  chapter  we  are  tolerably  sure. 
Their  truth  is  commensurate  with  that  of  the  em- 
bryological  argument  in  general,  and  if  we  accept 

87 


88  THE   LIVING  WORLD. 

the  teachings  of  embryology  as  indicating  the  past 
history  of  animals,  we  may  regard  the  general  facts 
of  the  derivation  of  the  Gastrasa  from  the  unicellu- 
lar animals,  and  the  subsequent  modification  of  its 
descendants  into  the  different  sub-kingdoms  which 
soon  filled  the  world,  as  substantially  proved. 

We  come  now  to  the  ages  represented  by  fossi- 
liferous  rocks,  and  immediately  the  record  is  much 
more  definite.  When  we  strike  the  fossiliferous 
rocks  we  seem  to  be  dealing  with  a  more  tangible 
subject.  The  animals  which  were  buried  in  the 
ancient  muds  and  sediments,  and  which  we  are  now 
unearthing,  were  actual  animals  and  not  hypotheti- 
cal ones.  With  all  of  the  cogency  of  argument  from 
the  embryological  standpoint,  it  cannot  be  denied 
that  the  steps  in  the  history  of  animals  which  it 
points  out  are  more  or  less  hypothetical,  and  the 
stages  in  the  history  of  life  of  which  it  speaks 
are  stages  of  type  only.  But  when  we  take  a 
fossil  in  our  hands,  we  cannot  question  that  the 
fossil  was  once  an  actual  animal,  and  an  inhabitant 
of  the  world  in  the  earlier  ages.  It  is  now  possible, 
moreover,  to  determine  many  details  in  regard  to 
early  life,  for  by  means  of  fossils  we  deal  with  actual 
animals,  and  not  simply  with  structural  types.  For 
reasons  already  pointed  out,  however,  it  is  still 
impossible  to  obtain  connected  history. 

The  Geological  Ages. 

Before  proceeding  with  the  history  of  animals  as 
taught  us  by  fossils,  it  will  be  necessary  to  summarize 
briefly  the  geological  ages.  The  accompanying  figure 


Quarternary  do) 


Tertiary  (9)  - 


Cretaceous  (8)  . 


Jurassic  (7)  - 


Triassic(6)  -j 
Permian  (5)  •{ 


Carboniferous  (4)  * 


Devonian  (3) 


Silurian  (2)  - 


Archean  (i) 


Recent 

II 

Piocene 

Miocene 

Eocene  Mammals 

Birds 

Reptiles 
/ 

Reptiles 

Amphibia 
Coal  Measures 

Sub  Carboniferous 

Fishes 


Cenozoic 


Mesozoic 


Upper  Silurian 

Vertebrates 
Lower  Silurian 


Invertebrates 


Cambrian 


Paleozoic 


No  life 


FIG.  9. — Diagram  illustrating  a  section  of  the  earth's  crust  to  show  the  sue- 
cession  of  the  geological  ages. 


go  THE  LIVING  WORLD. 

(Fig.  9)  represents  the  ages  in  the  order  of  their 
occurrence.  In  the  subsequent  pages  the  ages  will 
be  referred  to  by  the  names  given  in  the  left  hand 
column  in  the  figure,  and  for  convenience  of  the 
reader  who  may  not  be  familiar  with  them,  a  num- 
ber will  be  placed  after  the  name  of  the  age  which 
will  indicate  its  relative  order.  Thus  Archean  (i)  in- 
dicates  that  the  Archean  age  was  the  first  of  the 
geological  ages. 

The  following  brief  description  of  the  life  of  the 
different  ages  will  serve  as  an  introduction  to  their 
more  careful  study  : 

PALEOZOIC. 

The  Archean  (i)  is  characterized  by  having  no 
traces  of  life.  This  does  not  mean  that  there  was 
no  life  on  the  world  ajhthat  time,  but  either  that  the 
conditions  were  not  favorable  for  the  preservation  of 
fossils,  or  that  the  subsequent  changes  in  the  rocks 
(metamorphosis)  have  obliterated  them.  There  is 
every  certainty  that  life  must  have  been  in  exist- 
ence, but  no  fossil  remains  of  animals  assist  us  to 
this  conclusion.  All  of  the  history  dwelt  upon  in  the 
last  two  chapters  must  have  occurred  during  the 
Archean  age,  for,  with  its  close,  all  of  the  early 
history  had  been  past. 

The  Silurian  (2)  is  characterized  by  the  presence 
of  animals  in  great  profusion.  The  animals  charac- 
teristic of  the  age  were  the  invertebrates  ;  for  while 
it  is  certain  that  vertebrates  were  in  existence  before 
its  close,  they  were  but  slightly  developed  in  com- 
parison with  the  invertebrates,  and  did  not  appear  at 


THE   RECORD   FROM   FOSSILS.  $1 

its  beginning,  at  least  so  far  as  present  evidence 
goes. 

The  Devonian  (3)  is  characterized  by  the  appear- 
ance of  the  vertebrates  in  large  numbers.  It  is  fre- 
quently called  the  age  of  fishes,  from  the  abundance 
of  these  animals.  No  higher  vertebrates  are  known 
to  have  been  in  existence. 

The  Carboniferous  (4)  was  an  age  characterized  by 
its  abundant  vegetation.  It  was  during  this  age  that 
most  of  the  coal  beds  were  deposited.  The  most  im- 
portant additions  to  animal  life  were  the  Amphibia, 
and  probably  the  Reptilia.  The  Amphibia  were  large 
animals,  and  were  especially  abundant. 

MESOZOIC. 

The  Permian  (5)  was  an  age  of  which  only  slight 
records  are  left.  It  was  an  age  in  which  important 
changes  in  the  animal  kingdom  took  place.  The  true 
Reptilia  became  more  abundant,  and  it  is  probable 
that  at  this  period  the  Mammalia  first  separated 
from  a  reptilian  stock. 

The  Triassic  (6)  was  especially  characterized  by  the 
development  of  numerous  reptiles,  some  of  great  size. 

The  Jurassic  (7)  again  found  the  reptiles  the 
most  prominent  animals.  New  reptiles  appeared, 
many  of  them  being  of  immense  size.  At  this  time 
we  find  the  first  indication  of  the  birds,  suggested 
by  many  bird-like  reptiles.  The  first  real  feathered 
animal  also  appeared,  a  bird  with  remarkable  reptile- 
like  characters.  In  this  age  the  reptiles  reached  their 
highest  development. 

The   Cretaceous   (8)  was  characterized  by  a  slight 


92  THE  LIVING  WORLD. 

diminution  in  the  abundance  of  reptiles,  though 
they  were  still  the  predominant  type.  Birds  more 
closely  allied  to  our  modern  birds  began  to  appear. 

CENOZOIC. 

The  Tertiary  (9)  began  the  modern  era.  Modern 
birds  were  found  in  abundance,  and  the  reptiles 
lost  their  prestige.  True  mammals  (in  distinction 
from  the  marsupials)  made  their  appearance  either 
at  this  time  or  slightly  earlier.  During  the  Tertiary 
they  rapidly  developed  into  the  orders  which  fill  the 
world  to-day,  and  almost  immediately  became  the 
predominant  type. 

The  Quart ernary  (10),  the  age  of  man,  brings  us  to 
the  present  time. 

These  long  periods  were  of  immense  duration,  but 
we  have  no  means  of  determining  even  approximately 
how  long  they  lasted.  Nor  do  they  by  any  means 
represent  the  whole  of  geological  time.  The  geo- 
logical history  was  doubtless  a  continuous  one,  and 
if  we  had  in  our  possession  the  whole  of  the  history 
we  should  hardly  have  been  able  to  divide  it  into 
ages.  The  periods  outlined  above  seem  very  distinct 
from  each  other  because  they  are  separated  by  lost 
periods  of  which  no  record  remains  to  us.  Between 
each  of  the  periods  above  mentioned  there  is  a  break 
in  our  record  representing  the  periods  during  which 
most  important  changes  took  place  in  the  history 
of  life,  but  of  which  we  can  never  obtain  any  knowl- 
edge. We  do  not  even  know  whether  the  lost 
periods  were  longer  or  shorter  than  those  of  which 
we  have  record,  but,  judging  from  the  amount  of 


THE  RECORD  FROM  FOSSILS.  93 

change  that  took*  place  in  animal  life,  they  must 
have  been  at  least  as  long.  Owing  to  these  lost 
records  it  frequently  happens  that  the  opening  of 
the  different  periods  shows  a  surprising  acquisition 
of  the  new  forms  of  life.  The  sudden  appearance  of 
new  types  of  life  at  the  opening  of  the  Tertiary  (9), 
for  example,  is  doubtless  due  to  the  fact  that  we  do 
not  possess  the  history  of  the  immediately  preceding 
period  ;  and  so  also  the  sudden  appearance  of  life 
with  the  Silurian  (2)  is  in  the  same  way  due  to  the 
absence  of  any  record  of  pre-Silurian  life. 

Life  at  the  Beginning  of  the  Fossiliferous  Record. 

We  will  now  turn  our  attention  more  closely  to  the 
development  of  life  during  these  ages.  In  the  first 
place  it  must  be  noticed  again  that  it  is  no  longer 
possible  to  trace  the  history  of  life  along  a  single 
road.  The  Gastraea  was  the  last  point  which  the 
various  types  of  animal  life  had  in  common.  From 
this  point  many  of  the  types  diverged,  and  the  com- 
plete study  of  the  history  of  life  would  lead  us  to 
follow  each  of  them.  This  would,  however,  involve 
a  mass  of  detailed  statistics  which  would  be  largely 
unintelligible  to  the  general  reader.  Instead  of  fol- 
lowing the  history  of  life  through  its  numerous  de- 
tails, we  shall  try  therefore  to  take  a  perspective 
view  of  it ;  noting  the  general  truths  and  laws  illus- 
trated by  the  course  of  development. 

Let  us  retrace  our  steps,  and  study  more  carefully 
the  history  of  life  during  the  geological  ages.  First 
we  may  ask,  What  was  the  condition  of  the  animal 
kingdom  at  the  time  of  our  first  fossil  record  of  it  ? 


94  THE  LIVING  WORLD. 

Upon  looking  at  the  world  during  the  Silurian  (2) 
age,  we  are  immediately  struck  with  the  surprisingly 
great  diversity  of  its  animal  life.  The  divergence  of 
types  of  animals,  for  which  we  have  found  evidence 
in  the  embryology,  had  already  taken  place.  The 
Gastraea  had  given  rise  to  a  number  of  lines  of 
descendants,  all  of  which  had  more  or  less  com- 
pletely lost  the  original  Gastraea  characters,  and  had 
taken  the  characters  of  the  animal  types  of  to-day. 
Not  only  had  the  large  types  made  their  appearance, 
but  there  had  already  been  time  enough  for  most  of 
them  to  diverge  still  further,  and  produce  their  char- 
acteristic classes. 

The  diversity  of  life  found  in  the  Silurian  (2)  age 
has  greatly  surprised  geologists.  This  surprise  is, 
however,  disappearing,  as  we  learn  from  embryology 
of  the  rapid  divergence  of  types  at  the  beginning  of 
life,  and  as  we  remember  the  long  blank  Archean  (i) 
age,  during  which  life  existed  without  leaving  any 
distinct  traces  of  itself.  It  must  be  remembered 
also  that  the  Silurian  (2)  age  itself  was  of  very  long 
duration,  and  many  of  the  forms  of  animal  life 
attributed  to  this  period  were  not  in  existence  at  the 
beginning  of  the  era,  but  were  developed  during  its 
progress.  The  scantiness  of  the  remains  makes  it, 
however,  impossible  to  determine  with  any  degree 
of  probability  which  of  the  animal  forms  came  in 
later.  In  our  study  of  this  early  period  of  life's  his- 
tory, we  will  consider  the  age  a  unit,  always  under- 
standing that  it  is  a  unit  of  immeasurable  duration, 
and  if  we  had  been  able  to  watch  its  progress,  many 
of  the  forms  which  appear  so  suddenly  would  have 


THE  RECORD   FROM  FOSSILS.  95 

been  found  to  be  slowly  developing  during  its  prog- 
ress. Before,  or  during  the  Silurian  age,  all  of  the 
sub-kingdoms  of  animals  came  into  existence. 
Ccelentera  were  present  in  the  shape  of  sponges, 
corals,  hydroids.  Among  Echinodermata  were  found 
crinoids,  star-fishes,  and  some  extinct  forms  of  sea 
urchins.  Various  mud  tracks  tell  us  that  marine 
worms  lived  in  these  early  mud  flats.  Mollusks 
were  abundant,  some  related  to  clams  and  oysters 
(lamellibranchs),  and  others  related  to  our  snails 
and  cuttle-fishes.  Brachiopoda  existed  in  marvellous 
numbers,  and  the  closely  allied  Bryozoa  were  plenty. 
Crustacea  there  Avere,  much  like  our  shrimps  and 
lobsters,  and  a  special  type  (trilobites)  was  highly 
characteristic  of  the  age.  Scorpions  were  present 
to  represent  the  air-breathing  animals,  and  the  exist- 
ence of  their  sting  tells  us  that  other  air-breathing 
animals  (probably  insects  of  some  kind)  had  also 
appeared.  Of  the  sub-kingdom  Vertebrata  the 
indications  are  scanty,  though  there  is  no  doubt  that 
they  were  in  existence  by  the  close  of  the  Silurian 
age,  for  in  the  upper  rocks  of  that  period  unques- 
tionable fossil  evidence  has  been  found,  and  recent 
discoveries  have  shown  them  in  the  lower  Silurian 
rocks,  thus  carrying  them  almost  to  the  bottom. 
The  earliest  vertebrates  could  not  have  had  any 
hard  parts  to  be  preserved,  true  bones  and  even 
cartilage  being  of  more  recent  origin,  and  this 
explains  their  scanty  remains  in  the  lowest  rocks, 
although  there  can  be  no  doubt  that  the  vertebrates 
must  have  been  more  or  less  abundant  even  in  the 
lower  Silurian  times. 


96  THE  LIVING  WORLD. 

If  we  examine  this  fauna  a  little  more  closely,  its 
high  state  of  diversity  appears  to  be  even  more 
startling.  Not  only  were  all  of  the  sub-kingdoms 
represented,  but,  omitting  the  vertebrates,  all  but 
two  of  the  classes  as  well.  Two  thirds  of  the 
orders  of  invertebrates,  and  quite  a  large  number  of 
the  families  which  are  in  existence  to-day,  were  then 
developed.  It  may  be  said  that  all  of  the  inverte- 
brates had  already  developed  their  large  trunks,  and 
the  subsequent  ages  have  in  most  cases  only  sufficed 
to  elaborate  the  minor  branching  of  the  trunks  thus 
formed.  The  Archean  (i)  age  had  been  sufficient 
for  the  divergence  of  the  types  of  which  embryo- 
logical  teachings  have  so  plainly  given  evidence. 

A  few  figures  will  illustrate  the  fact  just  men- 
tioned, that  the  divergence  of  type  before  the  Silurian 
(2)  age  produced  greater  modifications  than  those 
that  have  occurred  since.  Of  the  sub-kingdoms  of 
animals,  all  were  in  existence  during  the  Silurian. 
Of  next  smaller  groups,  the  classes,  all  were  in 
existence  whose  hard  parts  enabled  them  to  be  pre- 
served, with  the  exception  of  one  or  two  small  classes, 
whose  shell  is  but  poorly  adapted  for  preservation. 
Of  the  orders,  of  animals  again,  that  have  left  any 
fossil  records,  thirty-four  are  found  in  the  Silurian 
rocks,  nineteen  being  in  the  Primordial,  (i.  e.,  at  the 
very  bottom  of  the  series),  while  only  twenty-five 
orders  have  appeared  in  the  more  recent  rocks. 
Since  some  of  these  twenty-five  orders  have  only 
slight  skeletons,  their  preservation  must  be  regarded 
as  accidental,  and  their  absence  from  the  Silurian  (2) 
rocks  does  not  prove  that  they  did  not  then  exist. 


THE  RECORD  FROM  FOSSILS.  97 

Nine  of  the  later  appearing  orders  were  insects  which 
fed  upon  flowers,  and  could  only  have  appeared 
after  flowering  plants  ;  and  in  general  we  notice  that 
the  later  appearing  orders  were  largely  land  animals. 
Besides  the  orders  included  in  the  above,  there  are 
thirty-four  which,  from  the  soft  structure  of  their 
body,  have  left  no  traces  in  the  rocks  at  any  period. 
Of  these,  it  is  certain  that  a  number  at  least  must 
have  been  in  existence  in  the  Silurian  (2),  since  their 
close  allies  are  known  to  exist  there.  Carrying  our 
study  into  smaller  groups,  and  including  now  sub- 
orders in  our  figures,  we  find  the  later  ages  coming 
out  in  more  prominence.  Of  the  one  hundred  and 
five  orders  and  sub-orders  that  have  left  any  evidence 
of  themselves  in  the  rocks,  forty-four  are  Silurian 
and  sixty-one  have  appeared  in  the  later  ages.  Here 
too  we  find  that  of  the  sixty-one  later  sub-orders 
many  are  terrestrial,  and  at  least  twenty  are  insects-. 
It  is  further  noticed  that  the  larger  number  of  sub- 
orders which  are  of  recent  origin  belong  to  the 
higher  rather  than  the  lower  orders  of  animals. 
Taking  families  into  consideration,  a  larger  number 
were  late  in  appearing,  though  a  number  of  our 
modern  families  date  from  the  Silurian  (2). 

Thus  we  see  that  the  Archean  age  produced  the 
sub-kingdoms,  the  classes,  and  most  of  the  orders  of 
animals,  while  the  subsequent  ages  have  only  pro- 
duced the  smaller  divisions,  giving  rise  to  a  far  less 
divergence  in  type,  but  a  much  larger  profusion  of 
minor  branches.  Now  from  all  this  it  follows  that 
the  study  of  fossils  is  unable  to  help  us  to  the  knowl- 
edge of  the  early  history  of  any  of  the  sub-kingdoms 
7 


98  THE  LIVING  WORLD. 

except  the  vertebrates,  since  practically  all  of  them 
had  developed  before  the  fossil  record  began.  It  is 
true  that  in  the  subsequent  ages  most  of  the  other 
sub-kingdoms  have  very  much  expanded  in  variety 
of  forms,  and  have  in  general  seized  upon  larger  and 
larger  fields  in  nature.  But  in  most  of  them  the 
real  advance  has  been  slight,  and  in  many  there  has 
been  actually  no  advance,  but  only  production  of 
new  genera  and  species.  In  the  vertebrates  alone 
has  all  of  the  development  taken  place  during  the 
period  of  which  we  have  fossil  record,  and  here  alone 
can  we  read  a  definite  history  of  progression,  since 
here  alone  can  we  trace  anything  like  a  continuous 
history. 

With  the  opening  of  the  Silurian  (2),  therefore, 
the  animal  kingdom  bursts  upon  us  in  a  compara- 
tively high  state  of  development.  From  this  time 
on  there  is  a  constant  widening  of  types,  a  constant 
succession  and  disappearance  of  old  forms  and  the 
appearance  of  new  ones. 

As  already  seen,  the  Gastraea  was  the  last  point  in 
the  history  of  life  shared  in  common  by  all  multicel- 
lular  animals.  From  this  point  there  was  a  parting 
of  the  ways,  and  it  is  therefore  no  longer  possible  to 
follow  the  history  of  the  animal  kingdom  as  a  whole. 
It  will  be  necessary  to  take  up  the  different  branches 
and  follow  them  separately.  Of  course,  the  further 
we  trace  them  the  greater  and  more  complex  will 
become  the  branching,  but  it  is  not  our  intention  to 
trace  this  history  in  very  great  detail.  It  will  be  neces- 
sary, however,  here  to  take  into  brief  consideration 
the  history  of  the  various  sub-kingdoms  of  animals 


THE  RECORD  FROM  FOSSILS.  99 

as  they  are  known  to-day.  Inasmuch  as  this  will 
necessarily  lead  to  considerable  detail  which  will  be 
uninteresting  to  the  general  reader,  this  part  of  our 
subject  may  be  omitted  without  any  break  in  con- 
tinuity. 

PROTOZOA. 

The  unicellular  animals  are  all  minute,  and  most  of  them  are 
entirely  soft.  For  these  reasons  they  are  not  well  adapted  for 
preservation  as  fossils,  and  only  one  of  the  three  classes  has  been 
preserved  in  the  rocks  to  any 'great  extent. 

The  Foraminifera  have  usually  a  shell  of  lime,  and  therefore  have 
left  traces  of  themselves  in  all  ages.  Even  in  the  Archean  (i)  there 
is  a  curious  body  (Eozoan  Canadense)  believed  by  some  to  be  the 
remains  of  Foraminifera,  though  it  is  usually  regarded  to-day  as  a 
mineral  deposit.  With  the  Silurian  (2),  however,  true  foraminifers 
appeared  unquestionably  in  abundance,  and  every  subsequent  age 
shows  traces  of  their  presence.  There  are  two  geological  periods  in 
which  their  deposits  are  of  special  importance.  The  immense  chalk 
beds  of  Cretaceous  (8)  are  made  very  largely  of  shells  of  foraminifers. 
This  chalk  is  almost  exactly  the  same  in  its  formation  as  the  so-called 
Globergerina  ooze  that  is  being  deposited  now  at  the  bottom  of  the 
ocean,  so  that  it  seems  that  chalk  is  still  being  formed  in  our  modern 
seas.  The  second  large  deposit  of  foraminifers  is  the  Numulitic  lime- 
stone of  the  Tertiary,  an  immense  bed  of  European  rocks  covering 
thousands  of  miles  of  territory.  We  must  not  conclude  that  in 
these  two  periods  the  Foraminifera  were  any  more  abundant  than  in 
others,  but  simply  that  the  conditions  were  more  favorable  for  their 
preservation.  It  is  remarkably  interesting  that  the  species  existing 
in  the  Tertiary  (9)  and  many  of  those  of  the  Cretaceous  (8)  chalk  are 
identical  with  species  found  living  to-day,  and  when  we  go  still 
further  back  in  history  the  amount  cf  change  is  very  slight.  Indeed, 
Dr.  Carpenter  says  there  is  no  evidence  of  any  fundamental  modifica- 
tion or  advance  of  the  foraminiferous  type  from  the  Paleozoic  period 
to  the  present  time. 

A  second  order  of  the  Protozoa,  the  Radiolaria,  possess  a  skeleton 
of  silica  which  is  tolerably  well  adapted  for  preservation.  These 
have  been  traced  back  as  far  as  the  Silurian  (2),  though  it  is  not  until 


100  THE   LIVING  WORLD. 

we  reach  the  Mesozoic  (5,  6,  and  7)  rocks  that  their  remains  become 
abundant. 

The  other  Protozoa,  having  no  hard  parts,  could  not,  of  course, 
have  been  preserved  as  fossils.  There  can  be  no  doubt  now  of  their 
existence  through  all  of  the  geological  ages,  since,  as  we  have  seen, 
the  Foraminifera  and  Radiolaria,  which  are  the  highest  of  the  Protozoa, 
have  lived  during  all  these  periods,  and  the  lower  orders  of  any  group 
always  appear  before  the  higher  ones.  In  general,  then,  we  may 
conclude  with  little  chance  of  error  that  the  Protozoa  were  in  exist- 
ence at  the  beginning  of  the  Silurian  (2),  in  practically  the  same 
condition  that  they  are  now,  and  that  the  long  ages  since  have  seen 
very  little  modification  in  their  structure.  They  have  been  practically 
stationary,  and  their  development  was  almost  wholly  pre-Silurian. 

CCELENTERA. 

Four  classes  of  animals  are  to-day  included  under  the  head  of 
Ccelentera. 

Port/era  (sponges). — These  are  the  lowest  existing  multicellular 
animals.  As  would  be  expected,  therefore,  they  are  very  ancient  in 
origin.  They  are  found  in  the  lowest  Silurian  (2)  rocks  and  have 
been  found  in  every  age  since  that  time.  Here  also  the  probability 
seems  to  be  that  the  sponges  of  early  times  were  much  like  those  of 
to-day,  so  that  there  has  been,  very  little  real  advance  in  structure. 
The  variety  of  type  was  however  much  smaller  in  early  times  than 
to-day. 

Hydrozoa. — This  class  includes  the  hydroids  (see  Fig.  7)  and  the 
well-known  jelly-fishes.  The  jelly-fishes  being  so  soft,  it  is,  of 
course,  impossible  to  say  when  they  really  appeared,  their  absence 
as  fossils  proving  nothing.  The  first  traces  we  have  of  them  are 
in  the  Jurassic  (7).  Many  of  the  hydroids,  however,  have  a  shell, 
either  of  lime  or  of  some  horny  material,  and  they  have  been  found 
in  all  ages.  The  Silurian  (2)  rocks  contain  them  in  abundance.  But 
the  hydroids  of  this  period  were  quite  unlike  those  found  at  the 
present  day.  One  entire  class  found  in  the  greatest  abundance  at 
that  time  has  since  entirely  disappeared  (Graptolitoided),  the  last  traces 
being  found  in  the  Silurian  (2).  Several  smaller  groups  have  also 
disappeared.  Of  our  modern  forms,  only  one  class  (Thecophora)  was 
in  existence,  and  this  was  quite  different  from  its  representatives 
to-day.  This  class  continued  with  little  change  during  the  whole  of 


THE   RECORD  FROM  FOSSILS.  IOI 

the  Paleozoic  (2-4)  age,  and  it  was  not  until  the  later  part  or  the 
Mesozoic  (5-8)  that  the  modern  forms  began  to  appear,  and  the  class 
assumed  its  present  condition.  How  far  this  late  origin  of  most  of  our 
existing  groups  is  due  to  the  lack  of  preservation  of  animals  which 
really  existed  in  the  early  ages,  cannot  be  positively  determined.  The 
great  probability  is  that  most  of  our  modern  orders  are  much  older 
than  their  fossils  would  lead  us  to  believe. 

Aciinozoa  (this  includes  the  corals  and  the  sea  anemones). — The 
corals  have  commonly  a  lime  skeleton,  and  are  quite  well  adapted  for 
preservation.  They  have  been  very  abundant  in  all  ages.  The 
Silurian  (2)  rocks  contain  five  of  our  modern  orders,  though  most  of 
the  corals  were  quite  unlike  their  modern  representatives.  The  orders 
most  abundantly  represented  at  that  time  are,  however,  not  the 
orders  most  abundant  to-day.  During  all  the  Paleozoic  (2-4)  they 
continued  to  live  in  abundance,  a  large  variety  of  forms  being  in 
existence  all  through  that  time.  With  the  Mesozoic  (5-8)  however, 
we  find  the  modern  forms  of  corals  becoming  more  abundant,  and 
from  that  time  the  production  of  the  modern  coral  was  a  matter 
of  slow  and  constant  growth. 

The  Ctenophora  are  wholly  unrepresented  as  fossils,  owing  to  their 
soft  jelly-like  composition. 

In  general,  then,  the  Coelentera  form  an  ancient  group,  appear- 
ing in  abundance  in  the  earliest  rocks,  and  being  numerous  in  all 
ages.  During  the  Paleozoic,  however,  the  types  represented  were 
not  those  most  abundant  to-day,  and  many  of  them  were  confined  to 
the  Paleozoic  (2-4).  The  modern  Coelentera  seemed,  so  far  as  our 
record  goes  to-day,  to  have  appeared  with  the  Mesozoic  (5-8),  some 
of  the  ancient  types  disappearing  entirely  at  that  time  and  others 
becoming  of  subordinate  importance.  At  this  time  also  there  was  a 
great  multiplication  of  sub-orders  and  families. 

ECHINODERMATA. 

This  sub-kingdom  is  divided  into  five  classes  :  Crinoidea,  Aster- 
oidea  (star-fishes),  Ophuroidea  (brittle  stars),  Echinoidea  (sea  urchins), 
and  Holothuroidea. 

Crinoidea  (stalked  echinoderms). — This  class  may  be  especially 
regarded  as  the  fossil  class  of  the  group,  for  although  some  are  still 
in  existence  to-day,  they  are  very  few  in  numbers  (only  eight  genera), 
and  mostly  confined  to  deep  seas.  In  early  geological  times  they 


102 


.     THE   LIVING  WORLD. 


were,  on  the  other  hand,  by  far  the  most  abundant  of  the  echino- 
derms.  With  the  beginning  of  the  Silurian  (2)  great  numbers  and 
varieties  were  found  in  existence.  The  class  was  then  divided  into 
three  orders  (Brachiata,  Blastoidea,  Cistoidea),  ail  of  which  reached  a 
high  degree  of  expansion  in  the  Paleozoic  (2-4).  After  thus  reaching 
a  culmination  they  gradually  become  less  numerous  and  continued 
to  diminish  until  to-day,  when  there  exists  only  the  small  number 


D 


FIG.  io.  Diagram  illustrating  the  history  of  the  Echinodermata. — The  letters 
on  the  left  indicate  the  different  geological  ages.  E  Echinoidea.  O  Ophiuroidea, 
A  Asteroidea.  H  Holothuroidea.  C  Crinoidea.  B  Blastoidea.  Cy  Cystoidea. 

above  mentioned.  Two  of  the  former  orders  disappeared  com- 
pletely, and  thus  these  that  are  left  are  simply  surviving  fragments  of 
a  once  predominant  type.  The  modern  crinoids,  which  are  in  some 
respects  different  from  the  older  ones,  made  their  appearance  in  the 
Triassic  (6). 

Asteroidea  (star-fishes). — This  class  was  also  in  existence  in  Si- 
lurian (2)  times.  The  type  of  star-fish  found  in  these  rocks  is  slightly 


THE  RECORD  FROM  FOSSILS.  1 03 

different  from  that  of  the  modern  animals.  The  latter  appeared, 
however,  in  the  Devonian  (3),  and  the  ancient  type  disappeared  with 
the  Paleozoic  (2-4).  The  modern  forms  of  star-fish  appearing  in  the 
Devonian  (3)  continued  with  little  modification  until  the  Jurassic  (7), 
when  the  more  strictly  modern  families  began  to  appear. 

Orphiuroidea  (brittle  stars). — The  brittle  stars  were  quite  abundant 
in  the  Silurian  (2),  some  of  the  genera  being  identical  with  those 
living  to-day.  Their  history  has  been  of  little  special  interest.  They 
have  simply  continued  to  expand  slowly  into  the  present  condition. 

The  Echinoidca  (sea  urchins). — The  sea  urchins  were  well  developed 
in  the  Silurian  (2)  rocks,  but  were  of  a  type  quite  distinct  from  the 
modern  forms  (having  more  or  less  than  twenty  rows  of  plates).  This 
order  of  Palechinoidea  continued  to  exist  during  the  Paleozoic  (2-4), 
practically  disappearing  with  its  close.  With  the  Mesozoic  (5-8)  the 
modern  urchins  (with  just  twenty  rows  of  plates)  appeared,  quickly 
expanding,  and  reaching  a  profuse  state  of  development  in  the 
Jurassic  (7)  and  Cretaceous  (8).  Since  then  they  have  been  on  the 
wane. 

Holothuroidea  (sea  cucumbers). — The  sea  cucumbers  have  either  no 
shells,  or  sometimes  a  few  calcareous  plates.  As  fossils  they  are 
known  only  by  specimens  of  these  plates  which  are  occasionally 
found.  No  traces 4)L them  are  found  earlier  than  the  Carboniferous  (4), 
though  the  difficulty  of  determining  their  presence  makes  it  not  im- 
probable that  they  existed  at  an  earlier  period. 

In  general,  then,  the  echinoderms  were  very  early  developed.  All 
of  the  classes,  with  the  possible  exception  of  the  holothurians,  were 
found  in  the  Silurian  (2),  and  were  as  radically  distinct  from  each 
other  then  as  they  are  now.  The  Paleozoic  forms  were  on  the  whole, 
however,  quite  distinct  from  the  modern  representatives.  The 
modern  types  appeared  during  the  Mesozoic,  and  the  group  is  at 
present  on  the  wane. 

MOLLUSCOIDEA. 

Under  this  head  are  included  to-day  two  groups  of  animals,  whose 
position  in  the  animal  kingdom  is  unsettled,  though  they  are  certainly 
related  to  each  other. 

Brachiopoda. — These  include  animals  externally  resembling  bi- 
valve mollusks.  To-day  they  are  few  in  numbers  and  do  not  form 
a  very  important  group,  but  in  earlier  times  they  were  very  abundant. 


104  THE   LIVING  WORLD. 

In  the  Silurian  (2),  indeed,  the  Brachiopoda  were  more  abundant  than 
any  other  group  of  animals,  and  the  age  is  therefore  sometimes  called 
the  age  of  Brachiopoda.  At  this  time  they  reached  their  culmination, 
and  have  been  declining  ever  since,  though  quite  a  number  of  them 
are  still  in  existence.  It  is  especially  interesting  that  some  of  the 
genera  existing  to-day  are  not  to  be  distinguished  from  those  of  the 
very  earliest  rocks.  (Lingula,  Terebratula.) 

Bryozoa  or  Polyzoa. — These  form  a  group  of  small  animals  not 
generally  familiar  except  to  students  of  natural  history.  They  are 
abundant  to-day  at  the  sea  shore,  and  usually  are  mistaken  for  plants 
by  those  unacquainted  with  them.  They  are  small  animals,  but  many 
of  them  have  a  calcareous  shell.  They  appeared  in  the  Silurian  (2), 
and  existed  with  no  special  change,  though  as  the  modern  era 
approached,  the  animals  gradually  assumed  more  the  type  of  the 
existing  Bryozoa.  Like  the  Brachiopoda,  they  form  a  fossil  order 
whose  chief  development  occurred  in  the  past,  although  they  have 
not  yet  become  so  widely  extinct. 

Mollusca. 

Of  all  animals,  we  have  the  most  complete  geological  record  of  the 
mollusks.  They  were  well  developed  as  a  group  at  the  beginning  of 
the  Silurian  (2),  and  therefore  their  fossil  record  cannot  tell  us  any- 
thing of  their  early  history,  but  of  their  history  since  the  Silurian  we 
have  a  very  full  account.  Their  hard  shells  and  aquatic  habits  have 
adapted  them  especially  well  to  preservation.  We  recognize  five 
classes  of  mollusks,  whose  history  has  been  as  follows : 

Lamellibranchia  (oysters,  clams,  etc.). — This  group  is  characterized 
by  having  two  shells,  and  is  common  both  in  fresh  and  salt  water. 
The  class  was  abundantly  represented  in  the  earliest  Silurian  (2), 
many  of  our  modern  families  being  already  in  existence.  Indeed, 
quite  a  number  of  the  genera  then  living  still  exist.  By  the  close  of 
the  Paleozoic  (2-4)  are  found  many  others  not  so  well  known.  With 
the  Mesozoic  (5-8)  there  was  a  change.  Many  of  the  old  forms  dis- 
appeared and  new  ones  took  their  places.  During  the  rest  of  the 
geological  ages  there  was  an  approach  to  the  modern  fauna.  The  old 
forms  did  not  entirely  disappear,  however,  and  some  of  the  oldest  fami- 
lies continue  to  exist  to-day.  The  Mesozoic  is  marked  by  the  develop- 
ment of  the  clams,  quohogs,  and  other  less  well-known  mollusks.  The 
modern  forms  have  especially  developed  what  is  known  as  a  siphon. 


THE  RECORD  FROM  FOSSILS. 


105 


In  general  the  Paleozoic  (2-4)  lamellibranchs  were  commonly  without 
siphons,  although  some  of  the  siphonate  type  even  then  existed  ;  the 
modern  forms,  on  the  other  hand,  are  siphonate  as  a  rule,  though 
many  of  the  asiphonate  forms  still  exist.  The  most  remarkable 
point  in  the  history  of  this  class  was  the  development  of  a  very  pecu- 
liar order  in  the  Cretaceous  (9).  This  order  (Rugosites)  is  very 


FIG.  ii.      Diagram   illustrating  the  history   of  the   Mollusca. — C  Cephalopoda. 
P  Pteropoda.     H  Heteropoda.     G  Gasteropoda.     L  Lamellibranchiata. 


unlike  any  other  mollusks,  and  it  suddenly  appeared  without  warning 
in  the  Cretaceous  and  disappeared  at  its  close. 

Gasteropoda  (snails). — The  history  of  this  class  is  quite  parallel  to 
that  of  the  last.  Beginning  with  the  Silurian  (2),  they  have  been  at 
all  times  very  abundant.  With  the  Mesozoic  (5-8),  the  older  forms 


106  THE  LIVING  WORLD. 

began  to  disappear  and  the  modern  type  became  abundant.  The  first 
water  and  land  snails  are  found  in  the  Carboniferous  (4). 

Pteropoda  (winged  mollusks). — This  is  a  little-known  class,  found 
to-day  chiefly  in  the  high  seas.  Pteropods  were  abundant  in  the 
Silurian  (2)  rocks,  the  genera  seeming  to  be  identical  with  those  of 
to-day.  The  group  has  remained  practically  unaltered  till  to-day, 
except  that  some  of  the  early  families  have  entirely  disappeared.  At 
no  time  has  the  class  been  of  much  importance  in  the  world. 

Scaphopoda  (tooth  shells). — This  is  also  a  small  and  unimportant 
class,  consisting  to-day  of  only  three  genera.  It  is  of  great  antiquity, 
however.  The  first  remains  are  found  in  the  Devonian  (3),  although 
the  class  probably  existed  long  before  that  era.  It  has  remained 
practically  stationary. 

Cephalopoda  (squids,  cuttle  fishes,  etc.). — This  is  by  far  the  highest 
of  the  mollusks,  and  it  has  at  all  ages  been  an  important  group.  The 
class  has  two  well  marked  orders,  one  having  four  gills  ( Tctra- 
branchiata),  and  the  other  two  gills  (Dibranchiata).  Beginning 
with  the  early  Silurian  (2),  the  Tetrabranchiata  were  quite  abundant. 
Fourteen  orders  were  then  in  existence.  They  gradually  increased 
in  abundance,  size,  and  complexity,  although  only  one  new  type  was 
introduced  during  the  Devonian,  and  no  others  until  the  Tertiary  (9). 
They  reached  a  very  high  state  of  development  in  the  Mesozoic  (8). 
From  that  time  they  rapidly  diminished,  and  to-day  only  a  single 
species  (Nautilus)  is  left  as  a  remnant  of  this  once  predominant 
type.  Nautilus  is  in  itself  interesting  as  an  example  of  a  long- 
persistent  species,  being  abundant  as  far  back  as  the  Silurian  (2). 
Just  before  the  tetrabranchs  reached  their  culmination  the  first  repre- 
sentative of  the  dibranchs  appeared  (Triassic).  These  have  con- 
tinued to  increase  until  the  present  time,  one  large  section  of  them, 
however,  disappearing  with  the  Cretaceous  (Belemitcs).  To-day 
the  class,  though  existing  in  great  numbers,  is  impoverished  as  to 
variety,  only  a  few  types  remaining. 

The  mollusks  in  general  will  thus  be  seen  to  be  a  very  old  group, 
just  about  as  well  differentiated  at  the  beginning  of  the  Silurian  (2) 
as  they  are  to-day.  The  long  ages  have  seen  them  always  in  abun- 
dance, and  have  witnessed  slow  changes  and  slight  progression. 
There  has  been  a  constant  expansion  of  the  group,  but  it  has  consisted 
in  the  multiplication  of  families  and  genera.  As  elsewhere,  the  be- 
ginning of  the  Mesozoic  (5-8)  saw  the  older  types  giving  way  before 
the  modern  ones. 


THE  RECORD  FROM  FOSSILS. 


JO/ 


Of  the  history  of  the  unsegmented  worms  we  know  nothing  through 
geology,  for  their  soft  bodies  have  never  been  preserved.  From  their 
anatomical  position  we  conclude  that  they  are  very  ancient  animals, 
and  have  doubtless  lived  through  all  geological  ages. 


CO        M   Br         A         PI 


FIG.  12.  Diagram  illustrating  the  history  of  the  Crustacea. — C  Cirrepedia. 
O  Ostracoda.  M  Macroura.  Br  Bracyura.  A  Amphipoda.  P  Phylocarida. 
/  Isopoda. 

ARTICULATA. 

This  large  division  of  animals  includes  two  provinces. 

Anarthropoda  (segmented  worms). — The  worms  have  soft  bodies,  not 
adapted  for  preservation  as  fossils.  Some  of  them,  however,  live  in 
tubes  of  lime  or  sand,  and  these  cases  are  often  preserved.  Occasionally 
we  learn  of  their  existence  by  their  tracks  on  the  ancient  mud.  From 
such  sources  we  know  that  they  were  in  existence,  well  differentiated, 


IO8  THE  LIVING  WORLD. 

during  the  Silurian(2)  age,  and  have  lived  continuously  until   now, 
probably  with  little  change. 

Arlhropoda, — This  province  again  consists  of  three  classes. 

1.  Crustacea  (crabs,  lobsters,  etc.). — With  the  Crustacea  we  meet 
for  the  first  time  a  class  of  animals,  a  considerable  portion  of  whose 
development  has  occurred  since  the  Silurian  (2).      Still  the  class  is 
an  old  one,  and  most  of  the  lower  orders  were  distinct  at  our  earliest 
record  of  fossils.     The  Cirrepedia  (barnacles),  Ostracoda  (water  fleas), 
Amphipoda  (sand  fleas),  Trilobita,  and  Euripteridca,  as  well  as  one 
order  (Phyllocarida)  which  seems  to  have  been  the  ancestor  of  the 
higher  Crustacea,  were  all  in  existence  during  the  Silurian  (2).     But 
the  trilobites  and  euripterids  attained  their  culmination  at  that  time, 
and  soon  after  disappeared  in  the  Carboniferous  (4).      The    higher 
orders  certainly  appeared  later.     The  Isopoda  (sow  bugs)  and  Phyllo- 
poda  first  appeared  in  the  Devonian  (3).    The  shrimps  and  lobsters  are 
found  in  the  Devonian  (3)  and  Carboniferous  (4),  the  crabs  probably 
are  in  the  Carboniferous.   Now  since  the  crabs  are  undoubtedly  derived 
from  the  lobster  group,  and  this  group  from  the  Phyllocarida  above 
mentioned,  we  have  in  these  orders  an  instance  where  we  can  trace 
by  fossils  the  origin  of  the  modern  orders  from  the  earliest  lower  types. 
The  Crustacea  have  always  been  abundant,  and,  with  the  exception  of 
a  few  orders  that  have  disappeared,  are  as  abundant  to-day  as  ever 
before.     The  higher  orders  indeed  were  never  so  diversified  as  at  the 
present. 

2.  A  rachnoida  (spiders  and  scorpions). — Under  this  head  are  found 
the  oldest  air-breathing  animals.     The  scorpions  were  in  existence  in 
the  later  Silurian  (2),  and  true  spiders  are  found  in  the  Carboniferous 
(4).     That  these  have  been  in  existence  during  all  the  later  ages  is 
therefore  certain,  though  only  scanty  records  have  been  found. 

Myriopoda  (thousand  legged  worms). — Being  mostly  land  animals, 
these  have  left  scanty  remains.  They  are  found  in  the  Devonian  (3), 
and  doubtless  appeared  earlier. 

Insecta. — Though  the  insects  form  about  five  sixths  of  the  animals* 
of  the  world  to-day,  their  habits  prevent  their  ready  preservation  as 
fossils,  and  our  history  of  the  group  is  quite  meagre.  Some  of  the 
lowest  orders  (cockroaches)  were  undoubtedly  in  existence  in  the  Si- 
lurian (2).  During  the  whole  Paleozoic  (2-4),  insects  were  abundant. 
All  of  them  had  a  greater  or  less  resemblance  to  the  cockroach,  al- 
though in  the  Devonian  (3)  and  the  Carboniferous  (4),  they  took  on 
features  that  allied  them  to  the  higher  orders  of  insects,  the  beetles, 


THE-  RECORD  FROM  FOSSILS. 


true  bugs,  and  dragon-flies  being  thus  foreshadowed.  With  the 
Mesozoic  (5-8)  the  higher  orders  appeared,  and  by  the  end  of  the 
Jurassic  (7),  the  modern  orders  of  insects  were  all  in  existence.  Of 
the  various  orders  the  higher  ones  appeared  last.  These  orders, 
since  they  feed  upon  flowers,  could  not  be  expected  until  flowers  them- 
selves appeared,  and  this  was  not  until  the  middle  of  the  Mesozoic 
(5-8).  [See  Chap.  VI.]. 


FIG.  13.     Diagram  of  air-breathing  Arthropoda. — /  Insecta.     A    Araneina. 
Ar  Arachnoida.     M  Myriopoda. 


The  insects  have  thus  developed  within  the  period  of  fossil  history. 
In  the  Silurian  they  were  represented  by  the  lowest  order  and  this 
order,  during  the  rest  of  the  Paleozoic,  gradually  diverged  in  the  sev- 
eral directions  which  gave  rise  in  the  beginning  of  the  Mesozoic  (5-8) 
to  the  beetles  and  bugs  and  dragon-flies.  Later,  when  the  flowering 
plants  made  their  appearance,  there  was  a  second  divergence  and  a 
production  of  new  forms  depending  upon  flowers  for  their  existence. 


110  THE  LIVING  WORLD. 

It  seems  probable  that  the  remarkable  social  habit  of  the  ants  and 
bees  did  not  develop  until  later  than  the  Cretaceous  (8). 

VERTEBRATA. 

/  The  earliest  traces  of  the  Vertebrata  are  in  the  rocks  of  the  lower 
Silurian  (2).  At  that  time  there  is  no  doubt  that  some  forms  of  fishes 
were  in  existence,  though  the  remains  are  rather  scanty.  The  verte- 
brates then  existing  were  low  fishes  whose  skeleton  was  poorly  adapt- 
ed to  preservation.  Indeed,  in  the  whole  Silurian  the  traces  of 
vertebrates  are  rare,  although  there  can  be  no  doubt  that  they  were  in 
existence.  With  the  Devonian  (3),  they  became  very  numerous.  The 
Devonian  seas  were  filled  with  large  numbers  of  fishes,  chiefly 
ganoids  and  elasmobranchs  (related  to  the  gar-pikes,  and  sharks). 
During  this  period  these  orders  of  fishes  became  very  abundant  and 
highly  diversified.  Late  in  the  Devonian  age  some  of  the  ganoids 
would  seem  to  have  acquired  the  habit  of  living  partly  in  the  air,  and 
the  habit  thus  acquired  gave  rise  to  the  amphibians,  which  appeared 
for  the  first  time  in  the  next  age,  the  Carboniferous  (4).  During  this 
age  the  amphibians  became  abundant,  and  towards  its  close  they 
seem  to  have  become  more  distinctly  aerial  and  to  have  ceased  to  be 
able  to  live  in  the  water.  This  produced  the  true  land  vertebrates. 
In  the  latter  part  of  the  Carboniferous  (4)  or  the  Permian  (5),  we  find 
that  true  reptiles  had  come  into  existence.  Having  once  assumed  a 
terrestrial  habit,  the  reptiles  rapidly  expanded  to  appropriate  the  large 
field  open  to  them,  and  in  the  next  two  ages  (Triassic  and  Jurassic) 
they  became  more  and  more  abundant,  and  grew  to  immense  size. 
While  these  first  land  vertebrates  were  thus  expanding,  one  side 
branch  of  them  gradually  acquired  wings  and  developed  into  birds, 
which  seem  to  have  appeared  first  in  the  Jurassic  (7)  and  Creta- 
ceous (8). 

From  the  Carboniferous  (4)  the  reptiles  had  been  constantly  expand- 
ing in  diversity,  in  number,  and  in  size.  But  with  the  Cretaceous  (S) 
they  were  slowly,  yet  surely,  giving  way  to  another  and  better  adapted 
type  of  land  vertebrates.  Way  back  before  the  Jurassic  (7)  period,  prob- 
ably in  the  Permian  (5),  one  of  the  early  generalized  types  of  reptiles 
seem  to  have  sent  off  a  branch  of  descendants  which  produced  their 
young  alive  instead  of  laying  eggs,  and  nourished  them  for  a  longer 
or  shorter  time  by  secretions  from  the  dermal  glands  of  the  mother. 
These  animals  were  at  first  small  and  probably  weak.  But  they 
continued  to  exist  during  the  age  of  reptiles,  as  a  comparatively  unim- 


THE  RECORD  FROM  FOSSILS. 


Ill 


portant  group.     It  would  seem  that  somewhere  toward  the  close  of 

the  Cretaceous  (8),  or   perhaps  earlier,  these  animals  made  a  further 

improvement  upon  their  method  of  producing  young.     They  devel- 

M  BRA  F 


C  , 


FIG.  14. — M  Mammals.     B  Birds.     R  Reptiles.     A  Amphibiane.     F  Fishes. 

oped  the  placenta  which  enabled  them  hereafter  to  carry  their  young 
in  the  uterus  till  it  was  well  developed.  The  placental  mammals  thus 
arising  proved  to  be  a  much  stronger  type  than  that  which  had  existed 


112  THE  LIVING  WORLD. 

before.  Perhaps,  loo,  the  reptiles  had  expended  the  most  of  their 
energy,  so  that  they  were  ready  to  retire  before  any  newcomers.  At  all 
events,  we  find  that  with  the  end  of  the  Cretaceous  (8)  or  the  begin- 
ning of  the  Tertiary  (9)  the  new  group  of  mammals  rapidly  took  the 
place  of  reptiles.  A  large  share  of  the  reptiles  disappeared  and  those 
that  remained  took  a  subordinate  position  compared  to  that  of  the 
new  monarchs  of  the  world.  With  extreme  rapidity  now  did  the 
mammals  develop.  In  no  other  instance  in  the  animal  kingdom  has 
development  been  so  rapid.  Expanding  into  many  types,  occupying 
constantly  new  fields  of  nature,  they  soon  began  to  assume  forms 
familiar  to  us,  and  our  modern  families  began  to  appear.  The 
mammal  fauna  thus  became  more  and  more  like  that  existing  to- 
day, until  by  the  end  of  the  Tertiary  (9)  the  mammals  of  the  present 
period  may  be  said  to  have  been  in  existence.  Finally  in  the  Quar- 
ternary  (io)  period  there  appeared  one  animal  who  seized  upon  a  new 
and  untried  field  of  nature  for  its  own.  This  new  field  was  that  of 
mind,  and  this  new  animal  soon  distanced  every  other  competitor  and 
became  immeasurably  superior  to  all  other  animals.  This  of  course 


CHAPTER    V. 

A  VIEW   IN    PERSPECTIVE. 

AFTER  this  outline  sketch  of  the  history  of  the 
animal  kingdom,  we  may  now  try  to  take  a  perspec- 
tive view  of  the  whole,  in  order  to  get  a  better 
understanding  of  the  true  significance  of  the  history. 
Details  of  science  are  of  little  interest  or  of  little 
significance  until  they  are  collected  into  groups  and 
formulated  into  laws  ;  and  a  bird's-eye  view  gives  us 
a  clearer  idea  of  relations  than  an  elaborate  study  of 
details. 

Perhaps  the  most  striking  point  which  forces  itself 
to  our  attention  is  one  already  noticed,  viz. :  the 
great  extent  to  which  the  development  of  the  ani- 
mal kingdom  had  taken  place  even  prior  to  the 
beginning  of  the  fossil  period.  In  the  outline  of  the 
history  of  animals  as  sketched  in  the  previous  chap- 
ter, it  has  been  seen  that  all  the  groups  of  animals 
except  the  insects  and  vertebrates  had  already 
reached  a  high  state  of  development  before  the 
Silurian.  In  many  of  them  there  has  been  almost 
no  change  since  that  time.  In  others,  the  change 
has  been  chiefly  in  the  addition  of  new  families  and 
genera  without  any  special  modification  of  type. 
8  113 


114  THE  LIVING  WORLD. 

Not  a  few  of  them  reached  their  culmination  early  in 
the  Silurian  (2),  and  soon  after  became  extinct.  In 
other  words,  the  sub-kingdoms  and  classes  of  ani- 
mals, and,  in  many  cases,  the  orders  and  families, 
were  as  distinct  from  each  other  in  the  beginning  of 
the  Silurian  (2)  as  they  are  to-day.  The  divergence 
of  types  had  fully  taken  place  prior  to  the  beginning 
of  our  fossil  record.  Except  in  the  one  division  of 
Vertebrata,  the  development  of  classes  and  orders 
must  be  learned  from  embryology  and  anatomy  ;  the 
study  of  fossils  is  entirely  inadequate  to  help  us  to 
any  connected  history. 

Paleozoic,  Mesozoic,  and  Cenozoic  Epochs. 

A  second  equally  striking  fact  is  that  in  the  long 
history  that  has  intervened  between  the  Silurian  (2) 
/  and  the  present  time  there  have  been  two  specially 
prominent  dates.  The  first  was  at  the  beginning  of 
the  Mesozoic  (5-8).  During  the  Paleozoic,  as  we 
have  seen,  various  types  of  animals  had  existed  in 
abundance  ;  but,  though  the  three  Paleozoic  ages 
were  of  an  immensely  long  duration,  the  modifica- 
tions of  type  and  the  production  of  new  families 
during  its  progress  were  comparatively  slight.  With 
the  beginning  of  the  Mesozoic,  however,  there  was  a 
remarkable  expansion  and  development  of  almost  all 
classes  of  animals.  Within  a  comparatively  short 
time  a  greater  expansion  into  new  forms  took  place 
than  had  occurred  during  all  of  the  Silurian,  Devon- 
ian, and  Carboniferous  taken  together.  A  glance  at 
the  diagrams  (Figs.  10-14)  will  show  this  expansion 
in  a  striking  manner.  The  fact  that  there  was  such 


A    VIEW  IN  PERSPECTIVE.  115 

a  general  development  of  new  forms  at  this  period 
would  indicate  that  some  highly  important  change 
in  the  condition  of  the  surrounding  living  world 
occurred  to  act  as  a  stimulus.  Although  it  is  im- 
possible to  say  positively  what  these  new  conditions 
were,  we  may  perhaps  find  a  partial  explanation  in 
the  reduction  of  the  amount  of  carbonic  acid  in  the  at- 
mosphere. During  the  Carboniferous  (4)  era,  vegeta- 
tion had  been  rapidly  at  work  drawing  the  CO2  from 
the  air  and  storing  it  up  in  the  form  of  coal.  Roughly 
speaking,  we  may  say  that  all  of  the  carbon  now 
stored  in  the  coal  beds  was  in  the  atmosphere  in  the 
form  of  CO 2,  previous  to  the  Carboniferous  age. 
Plainly,  the  withdrawal  of  this  CO3  by  the  Carbon- 
iferous vegetation  rendered  the  air  much  better 
fitted  for  the  terrestrial  life  of  animals,  and  it  is 
interesting  to  find  that  during  this  period  or  just 
after  it  the  first  air-breathing  vertebrates  came  into 
existence.  Though  this  may  not  be  a  sufficient 
explanation  for  the  whole  of  the  marvellous  ex- 
pansion of  type  in  the  early  Mesozoic,  and  especially 
of  marine  types,  we  are  nevertheless  probably  correct 
in  regarding  it  as  one  of  the  most  important  factors 
in  this  expansion.  At  all  events,  it  is  true  that  some 
impulse  acted  upon  the  animal  kingdom  at  about 
the  beginning  of  the  Mesozoic  which  produced  an 
exceptionally  rapid  advance  and  a  divergence  of 
form. 

It  is  especially  noticeable  that  this  era,  which 
was  so  marked  in  the  life  history  of  animals,  was 
not  an  era  of  especial  note  in  the  vegetable  king- 
dom. It  was  not  until  toward  the  Cretaceous  (8) 


Il6  THE  LIVING  WORLD. 

age  that  the  plant  world  received  a  similar  impulse, 
causing  it  to  expand  into  modern  forms. 

The  second  date  of  importance  in  the  history  of 
animals  was  at  the  beginning  of  the  Tertiary  (9). 
With  this  age  we  may  say  that  the  strictly  modern 
order  of  nature  began.  From  this  time  we  begin 
to  find  in  abundance  the  families  and  genera  of  ani- 
mals which  characterize  the  world  to-day.  Of  our 
existing  species  of  animals,  the  lower  ones  were  the 
first  to  appear.  Or  stated  in  other  terms,  the  lower 
species  of  animals  are  of  longer  duration  than  the 
higher  ones.  In  the  lowest  rocks  of  the  Tertiary 
(Eocene),  we  find  representatives  of  existing  reptiles 
living  contemporaneously  with  a  world  of  extinct 
mammals.  None  of  our  living  mammals  had  then 
appeared,  though  modern  reptiles  were  abundant. 
Of  the  mammals,  too,  the  lower  orders  approached 
the  modern  condition  first,  the  Insectivora  and  the 
Edentata  considerably  antedating  the  Primates. 
During  the  Tertiary,  the  approximation  toward 
modern  fauna  was  very  rapid.  Old  species  disap- 
peared, and  new  ones  appeared  leading  directly  up 
to  the  present  time.  The  new  era  inaugurated  by 
the  Tertiary  was  not,  however,  so  marked  as  that 
coming  in  with  the  Mesozoic,  for  except  in  the  class 
of  mammals  there  was  far  less  expansion  of  type 
than  occurred  in  the  early  Mesozoic.  Still  the  new 
era  is  sufficiently  well  marked.  Of  the  causes  which 
produced  it  we  have  no  satisfactory  knowledge. 


I 


A   Constant  Change  of  Species. 


Looking  over  the  whole  history  as  seen  by  our 
collections  of  fossils  to-day,  we  find  that  through  all 


A    VIEW  IN  PERSPECTIVE.  117 

this  long  series  of  ages  there  has  been  a  constant 
change  of  species.  Few  species  of  animals  have 
succeeded  in  remaining  in  existence  very  long ;  or,  it 
is  probably  better  to  say,  few  species  of  animals 
have  succeeded  in  breeding  true  for  a  very  long 
time.  We  are,  of  course,  speaking  geologically,  and 
thus  are  reviewing  time  by  ages,  rather  than  by 
years.  Sooner  or  later  the  descendants  of  any  one 
species  died  or  became  so  changed  from  their  earlier 
form  as  to  constitute  a  new  species.  Thus  through- 
out the  past  new  species  have  been  continually 
appearing.  It  is  indeed  very  seldom  that  any  spe- 
cies continues  to  exist  from  one  age  to  another,  and 
thus,  as  a  rule,  the  species  of  each  age  were  distinct. 
As  already  noticed,  there  were  long  periods  between 
the  geological  ages,  during  which  no  record  of  life 
has  been  preserved.  These  blank  periods  were  un- 
doubtedly periods  of  considerable  disturbance  in 
nature,  and  they  lasted  many  thousands  of  years. 
The  only  record  we  have  of  the  events  that  occurred 
during  these  times  is  in  the  modification  which  they 
produced  upon  the  living  world.  The  disturbances 
were  usually  sufficient  to  change  animal  forms  so 
much  as  to  produce  an  entirely  new  set  of  species. 
It  is  thus  usually  possible  to  determine  closely  the 
age  of  any  fossil  by  its  specific  characters. 

(Nevertheless,  we  find  that  no  universal  rule  can 
be  given  ;  for  while  most  species  change  with  the 
advent  of  new  ages,  some  of  the  species  of  animal, 
were  much  more  persistent.  Some  forms  of  life 
remained  with  little  change  through  all  the  geologi- 
cal periods.  This  is  true  of  Lingula,  some  foramini- 
fers,  and  some  mollusks.  \  The  species  that  are  found 


Il8  THE  LIVING  WORLD. 

now  are,  to  be  sure,  different  from  those  found  in 
the  Silurian  (2),  but  the  genera  are  identical,  and  the 
differences  between  the  species  are  not  great.  In 
such  cases  the  same  species  may  exist  from  one  age 
to  another.  These  are.  however,  exceptional  in- 
stances. Generally  the  particular  species  character- 
istic of  each  age  were  replaced  in  the  subsequent 
age  by  a  new  set,  and  this  same  fact  is  also  true  of 
the  genera.  Families  were  longer  lived,  and  many 
families  living  in  the  Silurian  (2)  age  inhabit  our 
seas  to-day. 

It  is  not  common,  then,  for  any  species  to  succeed 
in  outliving  the  long  periods  between  the  geological 
ages.  Nevertheless,  the  different  species  of  animals 
have  had  quite  different  lengths  of  life.  Some  are 
confined  to  a  single  stratum  of  a  single  age,  while 
others  extend  through  two  or  three  geological  sys- 
tems (some  Brachiopoda).  It  is  found  also  to  be 
a  general  rule  that  the  lowest  species  are  of  the 
longest  duration.  For  instance,  certain  species  of 
Foraminifera  (Saccaminina  Carteri)  appeared  in  the 
Silurian,  and  have  continued  to  exist  until  to-day 
.with  little  or  no  modification.  So,  too,  with  the 
Lamellibranchiata  and  Crustacea,  the  lowest  forms 
appeared  early  and  continued  to  live  a  long  time, 
while  the  higher  ones  were  more  rapidly  modified. 
Again,  as  a  somewhat  better  illustration,  we  find  that 
in  the  Tertiary  (9),  the  reptiles  had  already  assumed 
the  forms  which  have  continued  to  exist  up  to  the 
present  time.  Not  so,  however,  with  the  higher 
vertebrates,  which  have  undergone  great  changes 
since  that  time.  The  same  is  true  everywhere.  Low 


A   VIEW  IN  PERSPECTIVE.  IIQ 

animals  with  simple  structure  are  not  so  delicately 
adapted  to  their  environment  as  to  require  a  change 
for  every  little  change  in  their  surroundings.  The 
higher  animals,  however,  are  so  complex  and  withal 
so  delicately  adjusted  to  their  environment  that  any 
little  change  of  conditions  throws  them  out  of  har- 
mony, and  requires  change  in  structure  to  meet  the 
new  conditions.  A  steam  engine  requires  much 
more  care  and  gets  out  of  order  more  readily 
than  a  water-wheel.  So  the  types  of  low  animals 
have  continued  to  exist,  while  the  higher  ones  have 
been  rapidly  modified. 

(7  It  is  to  be  noticed  further  that  not  only  has  this 
Change  of  species  been  a  constant  change,  but  that 
itrias  also  been  a  gradual  one.  There  have  proba- 
bly been  no  abrupt  breaks  in  the  history.  We  find 
that  when  one  species  disappears,  it  is  either  re- 
placed by  another^closely  related  form,  or  the  whole 
line  becomes  extinct.  In  all  cases  where  the  rocks 
have  preserveo!  for  us  anything  like  a  continuous 
history,  it  is  seen  that  there  are  no  abrupt  transi- 
tions from  one  type  to  a  radically  different  one.  In 
the  few  cases  where  the  record  is  exceptionally  com- 
plete, it  is  possible  to  trace  one  species  into  another 
by  innumerable  connecting  links.  Such  evidence 
is,  however,  rarely  forthcoming,  and  new  species 
commonly  seem  to  appear  abruptly.  /In  many  in- 
stances, indeed,  quite  new  types  of  life^eem  to  come 
suddenly  into  existence,  but  wherever  this  is  the 
case  we  find  that  there  is  usually  a  break  in  our 
record  of  history*.  Now  if  we  take  into  account  the 
long  periods  of  "'unrecorded  history  that  intervened 


120  THE  LIVING  WORLD. 

between  the  ages,  we  are  of  course  prepared  to  find 
that  each  age  is  characterized  at  its  outset  by  a  new 
set  of  species,  and  in  some  cases  by  a  distinctly 
new  fauna.  Remembering,  then,  that  all  breaks  in 
the  history  occur  where  there  are  breaks  in  the 
record,  and  that  there  are  no  breaks  in  the  history 
where  the  record  is  complete,  we  may  unhesitatingly 
conclude  that  the  real  history  of  life  has  been  one 
of  continual  slow  change,  without  breaks,  with  each 
age  passing  imperceptibly  into  the  next. 

Animal  Types  the  Same  To-Day  as  in  Earlier  Times. 

With  all  of  this  extinction  of  old  forms,  it  is 
remarkable  that  since  the  Silurian  (2)  no  great 
type  of  animals  has  disappeared.  All  of  the  sub- 
kingdoms  in  existence  in  the  Silurian  are  still  in 
existence  to-day,  and  the  same  is  equally  true  of  the 
classes  and  nearly  so  of  the  orders.  It  is  marvel- 
lously interesting  and  surprising  to  find  that  all  of 
the  fossils  found  in  the  rocks  deposited  so  many 
millions  of  years  ago  can  be  readily  placed  with  one 
or  another  of  the  divisions  of  animals  that  are  in 
existence  to-day.  The  fact  gives  us  a  forcible  lesson 
that  the  animal  kingdom  is  one,  and  that  during  all 
the  history  of  the  world  it  has  been  a  unit.  It  tells 
us  that  the  same  laws  in  force  to-day  were  in  force 
millions  of  years  ago  in  the  world.  It  indicates  that 
some  bond  has  united  the  animals  of  the  rocks  with 
those  of  our  seas  and  lands  ;  and  this  unity  gives  us 
one  of  the  strongest  arguments  for  the  belief  that 
heredity  and  evolution  have  been  the  laws  presiding 
over  the  development  of  the  animal  kingdom. 


A    VIE  W  IN  PERSPECTIVE.  121 

A  second  fact  of  equal  significance  is  that  not  only 
has  no  large  type  disappeared,  but  no  new  great  type 
of  animals  has  appeared  since  the  Silurian  (2)  age. 
This  has  been  sufficiently  discussed,  and  only  need 
to  be  mentioned  here  as  correlated  with  the  subject 
in  the  last  paragraph. 

During  the  geological  ages  species  differed  in 
their  range  as  they  do  to-day.  Some  were  confined 
to  small  localities  (certain  species  of  trilobites),  while 
others  had  an  almost  world-wide  range  (Foraminifera). 
As  would  be  expected,  the  species  with  a  long  range 
in  time  were  usually  the  ones  that  had  also  a  large 
geographical  range  (Saccuminina  Carteri).  To  this, 
however,  there  are  exceptions,  for  some  species  con- 
fined to  a  single  system  of  rocks  have  an  enormous 
range  in  geographical  area  (certain  ammonites). 

In  looking  over  the  whole  geological  period  we  find 
that  after  a  species  of  animals  has  once  disappeared 
it  has  never  again  reappeared,  no  single  instance 
being  known  which  would  serve  as  an  exception  to 
this  rule.  The  principle  may  be  carried  out  still 
further.  No  type  of  animals  is  ever  a  second  time 
developed.  It  is  probably  even  correct  to  say  that 
throughout  the  wide  animal  kingdom  no  organ  that 
has  fully  disappeared  is  ever  again  redeveloped. 
Animals  cannot  develop  a  second  time  that  which 
they  have  once  completely  lost ;  when  a  change  of 
circumstances  requires  an  animal  to  perform  again  a 
function  formerly  performed  by  a  lost  member,  the 
lost  member  is  not  redeveloped,  but  some  other 
organ  assumes  a  new  function.  For  instance,  the 
insects  once  possessed  many  legs  but  they  have  uni- 


122  THE  LIVING  WORLD. 

versally  lost  all  except  three  pairs.  Now  the  larvae 
of  butterflies  (caterpillars)  need  a  larger  number  of 
legs  than  this,  but  instead  of  redeveloping  the  lost 
legs  they  have  simply  had  a  fold  of  skin  modified 
to  serve  the  function  of  legs. 

Early  Forms  Intermediate  Between  Existing  Types. 

The  next  point  of  significance  which  we  notice  in 
the  examination  of  the  history  of  animals  is  that, 
although  the  earlier  animals  all  conform  to  types  still 
in  existence,  it  is  frequently  impossible  to  classify 
them  satisfactorily.  Among  the  invertebrates  there 
is  not  very  much  difficulty  in  this  respect,  probably 
because  of  the  fact  that  even  our  earliest  rocks 
contained  the  types  well  differentiated  from  each 
other.  When  we  study  the  early  representatives  of 
the  higher  types  whose  history  is  more  or  less  com- 
pletely represented  in  the  early  stages,  it  becomes 
difficult  and  indeed  impossible  to  determine  where 
to  place  them.  The  early  insects  of  the  Devonian 
(3)  and  Carboniferous  (4)  seem  to  be  related  to  sev- 
eral of  the  present  orders  of  insects,  and  it  is  impos- 
sible to  determine  whether  they  are  to  be  called 
beetles,  bugs,  and  dragon-flies,  or  are  to  be  regarded 
as  all  cockroaches  showing  an  approach  to  these 
other  types  of  insects.  Especially  is  this  difficulty 
of  classification  true  in  the  case  of  the  mammals. 
Among  the  early  representatives  of  this  class  of 
animals  in  the  Tertiary  (9)  some  are  so  truly  inter- 
mediate in  type  that  it  is  impossible  to  determine 
whether  they  were  insectivorans  or  marsupials,  while 
others  -stand  midway  between  the  carnivores  and 


A    VIEW  IN  PERSPECTIVE.  123 

ungulates.  In  short,  many  of  the  forms  found  in 
these  rocks  are  so  intermediate  in  type  between  the 
orders  which  we  find  in  the  world  to-day  that  they 
cannot  be  regarded  as  belonging  to  any  of  them,  and 
at  the  same  time"  they  show  so  many  characters  in 
common  with  the  mammals  now  existing  that  it 
is  equally  impossible  to  regard  them  as  forming  dis- 
tinct orders.  They  were,  in  fact,  intermediate  types. 
It  is  commonly  true  that  while  the  fossils  require 
no  new. types  created  for  their  classification,  the 
fossils  representing  the  introduction  of  any  group 
show  such  a  complication  and  combination  of  rela- 
tions that  it  becomes  impossible  to  classify  them 
satisfactorily  into  any  of  the  modern  orders.  As 
we  study  fossils  of  the  succeeding  rocks,  however, 
we  find  the  similarity  of  the  structure  to  the  modern 
orders  more  and  more  close,  so  that  the  nearer  we 
approach  the  present  >  time  the  easier  becomes  the 
task  of  fossil  classification.  The  interpretation  of 
this  is  of  course  found  in  the  fact  that  the  various 
orders  of  animals  have  separated  from  common 
centres  in  accordance  with  the  principle  of  descent, 
or  otherwise.  The  nearer  we  come  to  those  centres 
the  greater  is  the  similarity  of  the  animals  ;  it  was 
only  in  later  times  that  the  different  lines  of  descent 
became  sufficiently  separated  from  each  other  to  be 
recognized  as  distinct  orders. 

Introduction^  Development,  and  Decline  of  Types. 

We  must  now  notice  more  particularly  the  history 
of  some  representative  groups,  studying  their  method 
of  introduction,  expansion,  and  extinction.  In  gen- 


124  THE  LIVING  WORLD. 

eral  we  may  say  that,  at  the  beginning  of  their  history, 
all  groups  of  animals  are  few  in  numbers  and  of 
slight  variety  ;  then  they  expand  until  a  culmination 
is  reached,  when  they  begin  to  diminish  in  numbers 
until  they  reach  extinction,  and  are  replaced  by 
other  groups.  Such  seems  to  be  the  general  history 
of  all  groups.  Taken  separately,  however,  the  groups 


Q 

Te 

Cr 

Jr 

Tr 
P 
C 

D 


FIG.  15. 

differ  widely  from  each  other,  some  of  them  having 
become  extinct  long  ago,  and  others  not  yet  seeming 
to  have  reached  their  culmination.  Although  the 
history  of  the  different  orders  and  classes  is  almost 
as  varied  as  the  groups  themselves,  still  we  may 
recognize  several  distinct  types  of  development. 

Fig.  1 5. — Some  groups  of  animals  were  in  existence 
in  abundance  at  the  beginning  of  our  record  ;  they 


A    VIEW  IN  PERSPECTIVE.  12$ 

increased  rapidly  in  numbers  and  diversity,  soon 
reached  a  culmination,  and  then  disappeared  with 
an  almost  equal  rapidity  (see  Fig.  15).  The  tri- 
lobites,  for  instance,  were  numerous  in  the  Silurian 
(2).  During  this  age  they  became  highly  diversified, 
reached  their  culmination  in  the  last  of  the  same 
age,  and  then  rapidly  diminished  in  numbers  and 
disappeared.  The  same  is  true  of  euryptids.  The 
graptolites  were  abundant  in  the  Lower  Silurian  (2), 
reached  their  culmination  during  this  age,  and  dis- 
appeared completely  with  it,  no  traces  of  them  being 
found  in  any  subsequent  period.  Other  examples 
of  the  same  kind  are  the  Cystiphilloidea,  disappear- 
ing in  the  Devonian  (3),  the  Blastoidea  and  Cistoidea 
in  the  Carboniferous  (4),  and  also  the  Palaeechinoidea 
ending  their  history  in  the  Carboniferous  (4). 

Fig.  1 6. — Some  groups  not  in  existence  at  the  be- 
ginning of  the  history  seem  to  have  suddenly  appeared 
and  then  as  suddenly  disappeared  again  (see  Fig.  16). 
The  rugosites  of  the  Cretaceous  (8)  are  the  best  ex- 
amples of  this.  These  remarkable  mollusks,  so  unlike 
any  other  representatives  of  the  group,  seem  to  have 
become  quite  abundant  in  that  age.  They  completed 
their  history,  however,  with  the  Cretaceous,  and  no 
traces  of  them  subsequently  have  been  found.  None 
other  of  the  invertebrate  orders  had  a  similarly  short 
life.  One  order  of  the  dipnoids  (Acanthodini),  one 
of  Amphibia  (Stegecephala),  six  orders  of  reptiles 
(Ichthosauria,  Plesiosauria,  Pythonomorpha,  Thero- 
morpha,  Dinosauria,  and  Pterosauria)  have  been 
confined  to  comparatively  short  periods  in  the  past. 
Three  orders  of  birds  (Saururae,  Odontholcae,  Odon- 
totormae)  and  at  least  five  orders  of  mammals 


126  THE  LIVING  WORLD. 

(Toxodontia,  Condylarthra,  Amblypoda,  Mesotheria, 
Tillodontia)  are  confined  to  the  Tertiary  (9).  The 
duration  of  the  existence  of  the  above-mentioned 
orders  was  short.  In  many  cases  the  orders  were 
confined  to  one  system  of  rocks.  Of  course  the 
sudden  appearance  of  these  various  orders  is,  in  a 
measure,  misleading.  They  always  appear  after  a 


Q 

7> 
Cr 


Tr 

P 

C 

D 


FIG.  1 6. 

long  period  of  unrecorded  history,  commonly  at  the 
beginning  of  a  new  system  of  rocks,  and  this  fact 
must  be  interpreted  as  meaning  that  their  seemingly 
sudden  appearance  is  due  to  their  earlier  history 
having  been  lost.  In  some  of  the  orders  mentioned, 
indeed,  the  appearance  was  not  very  abrupt.  But, 
nevertheless,  all  of  these  groups  of  animals  are 
marked  from  others  by  their  rapid  development  and 
sudden  extinction. 


A   VIEW  IN  PERSPECTIVE. 


127 


Fig.  17. — Some  groups  have  appeared  in  greater  or 
less  abundance  in  the  Silurian  (2),  have  rapidly  cul- 
minated, and  then  slowly  dwindled  away,  but  have 
never  become  entirely  extinct,  even  up  to  the  present 
day  (see  Fig.  17).  The  crinoids  form  our  best  illus- 
tration. They  were  present  in  abundance  early  in 
the  Silurian  (2)  rocks,  they  rapidly  expanded,  reached 


FIG.  17. 

their  culmination  in  the  Carboniferous  (4),  and  have 
since  that  time  been  constantly  diminishing  in  num- 
bers. They  are  not  yet  extinct,  but  only  eight  genera 
of  this  once  predominant  type  are  known  to  exist, 
and  most  of  these  are  confined  to  the  depths  of  the 
sea.  The  Brachiopoda  also  were  so  abundant  in  the 
Silurian  (2)  age  that  they  may  be  called  the  charac- 
teristic animals  of  that  era.  Immediately  after  the 


128 


THE  LIVING  WORLD. 


Silurian  (2),  however,  they  began  to  diminish  in 
number,  and  have  been  growing  of  less  importance 
ever  since.  To-day,  though  not  so  much  diminished 
in  numbers  as  the  crinoids,  they  form  a  comparatively 
unimportant  group  with  few  genera.  The  orders 
of  Phyllocarida  form  the  only  other  important  illus- 
tration. This  is  a  small  group  of  Crustacea,  quite 
abundant  in  the  Silurian,  which,  with  the  exception 
of  a  single  genus  (Nebalia),  is  to-day  extinct. 


Q 

Te 
Cr 


FIG.  18. 

Fig.  1 8. — Some  groups  of  animals  appeared  well  de- 
veloped with  the  Silurian  (2)  age,  and  have  continued 
to  exist  in  undiminished  numbers  ever  since,  or 
have  even  increased  in  number  and  diversity  (see 
Fig.  1 8).  This  is  true  of  a  majority  of  the  orders 
found  in  the  Silurian.  Many  of  them  seem  to  have 


A   VIEW  IN  PERSPECTIVE. 


I29 


been  constantly  increasing  in  range  with  greater 
or  less  rapidity.  This  is  true  of  the  hydroids, 
corals,  star-fishes,  mollusks,  and  especially  of  the 
air-breathing  insects. 

Fig.  19. — Some  groups  not  present  at  the  beginning 
of  the  Silurian  appeared  subsequently,  perhaps  in 
small  numbers,  and  from  the  time  of  their  appearance 


FIG.  19. 

have  continued  to  expand  (see  Fig.  19).  Their  num- 
bers have  grown  larger  and  their  diversity  of  structure 
has  constantly  increased.  This  has  continued  until 
to-day,  and  now  they  exist  in  greater  abundance 
than  ever  before.  Under  this  head  are  included  all 
of  the  vertebrate  orders  living  to-day,  and  nearly  all 
of  the  insects,  at  least  of  the  higher  orders.  Among 
the  lower  animals  we  find,  it  true  of  the  dibranchiate 
9 


130  THE   LIVING  WORLD. 

Cephalopoda  (squids),  of  an  occasional  order  among 
the  rest  of  the  mollusks,  of  our  common  lobsters, 
shrimps,  and  crabs,  of  the  order  of  modern  sea- 
urchins,  and  seemingly  so  of  some  of  the  orders  of 
Coelentera  (Alcyonaria),  etc.,  though  of  this  we  can- 
not be  sure,  since  the  animals  appear  to  be  so  poorly 
adapted  to  preservation  that  their  early  history  is 
uncertain. 

It  is  plain  that  all  of  the  orders  of  animals  existing 
in  great  abundance  to-day  must  come  under  one  of 
the  two  classes  last  mentioned,  and  it  will  thus  follow 
that  nearly  all  of  the  animals  well  known  to  the  gen- 
eral reader  have  had  the  history  of  constant  expan- 
sion from  the  time  of  their  appearance  until  to-day. 

Fig.  20. — Not  infrequently  in  the  history  of  some 
groups  there  has  occurred  a  long  period  of  stationary 
condition,  followed  by  a  rapid  development  (see  Fig. 
20).  A  very  striking  illustration  of  this  is  shown  in 
Fig.  10,  which  represents  the  history  of  the  echino- 
derms.  From  the  figure  it  will  be  seen  that  the  sea- 
urchins  appeared  in  the  early  Silurian  (2),  but  during 
this  and  the  long  Paleozoic  ages  remained  practically 
stationary.  With  the  beginning  of  the  Triassic  (6), 
however,  some  influence  caused  the  urchins  suddenly 
to  begin  to  develop  into  new  forms.  In  a  very  short 
time,  comparatively,  there  were  then  produced  a 
large  number  of  sub-orders  and  families,  and  the  sea- 
urchins  were  brought  into  existence  practically  as 
we  have  them  in  the  world  to-day.  By  the  time  of 
the  Cretaceous  (8)  this  development  had  ceased,  and 
from  that  time  the  echinoid  group,  as  well  as  the  rest 
of  the  echinoderrns,  has  remained  stationary,  or  per- 


A   VIEW  IN  PERSPECTIVE. 


haps  has  even  been  declining.  The  mammals  again 
give  a  very  pretty  illustration  of  the  same  principle. 
This,  the  highest  class  of  vertebrates,  appeared  first 
in  the  Triassic  (6).  These  early  mammals  were  all 
small  animals  and  were  of  the  lowest  type,  the  mar- 
supials. During  this  and  the  two  subsequent  ages, 


FIG.  20. 

Jurassic  (7,)  and  Cretaceous  (8),  the  mammals  prob- 
ably continued  to  exist  with  very  little  change,  the 
few  remains  found  during  these  ages  not  certainly 
indicating  any  special  advance  over  those  of  the 
Triassic  (6).  At  the  close  of  the  Cretaceous  (8), 
there  seems  to  have  been  some  influence  acting 
upon  the  class  which  started  them  into  a  remarkably 
active  development.  Just  when  this  impulse  oc- 
curred, we  do  not  know,  but  with  the  beginning  of 


132  THE  LIVING  WORLD. 

the  Tertiary  (9)  we  find  abundant  fauna  of  .the 
higher  true  mammals,  indicating  that  by  this  time 
the  mammals  had  been  for  some  time  under  the 
influence  of  an  expanding  force.  From  this  early 
Tertiary  (9),  the  development  and  expansion  of 
the  mammals  occurred  with  great  rapidity  and  in 
a  very  short  time,  short  at  least  compared  with  the 
long  period  in  which  the  class  seems  to  have  re- 
mained dormant,  the  higher  mammals  had  diverged 
into  the  modern  ones. 

Causes  of  the  Sudden  Expansions  of  Type. 

The  question,  of  course,  arises  why  any  group  of 
animals  should  have  remained  so  long  in  a  compara- 
tively stationary  condition  and  then  have  suddenly 
expanded  with  such  rapidity  into  a  widely  diversified 
fauna.  It  is  hardly  ever  possible  to  give  a  definite 
answer  to  this  question.  A  somewhat  general  answer 
can,  however,  be  given,  which  is  very  suggestive 
as  indicating  one  of  the  important  laws  of  animal 
life.  It  would  seem  that  the  comparatively  sud- 
den expansion  of  type  has  occurred  when  a  group 
of  animals  in  some  way  begins  to  occupy  a  new 
field  of  nature.  Where  conditions  remain  constant, 
animal  life  is  held  in  comparative  equilibrium,  and 
therefore  is  more  or  less  stationary.  But  any  change 
in  conditions  will  disturb  the  equilibrium,  and  the 
result  is  always  a  change  in  the  structure  of  the 
animals  themselves.  Now  any  change  in  climate, 
in  temperature,  in  atmospheric  constitution,  in  con- 
figuration of  the  land  and  sea,  etc.,  will  be  sure  to 
affect  the  equilibrium  of  life.  That  such  changes 


A    VIEW  IN  PERSPECTIVE.  133 

have  been  constantly  occurring  in  the  past  we  have 
abundant  proof,  and  it  is  certain  they  always  induce 
changes  in  living  nature.  Indeed,  the  periods  of 
greatest  geological  disturbances  have  always  been 
those  of  most  rapid  evolution  of  animals  (e.  g.,  the 
end  of  the  Carboniferous  (4)  and  Cretaceous  (8).  It 
would  seem,  however,  that  more  restricted  causes 
may  frequently  have  been  the  origin  of  organic 
change.  A  group  of  animals  may  migrate  into  a 
new  territory,  and  finding  there  a  new  field  with  a 
less  powerful  set  of  competitors,  it  will  be  able  to 
expand  itself  to  a  much  greater  extent  than  in  the 
old  home.  That  this  is  a  result  of  such  migration  is 
well  known  from  the  study  of  animals  to-day.  Un- 
doubtedly such  local  changes  have  occurred  in  the 
past  and  have  constituted  one  of  the  factors  which 
have  caused  the  periodic  expansion  of  animals. 

There  is,  however,  another  factor  probably  of  even 
greater  importance,  but  one  which  cannot  be  so 
easily  understood  or  explained.  Internal  changes  in 
the  organism  itself  will  produce  new  fields  of  expan- 
sion. It  is  well  known  that  animals  are  constantly 
varying,  now  in  one  direction  and  now  in  another. 
These  variations  are  continuous,  and  seem  to  have 
no  direct  relation  to  any  change  in  environment. 
Of  the  many  variations  that  appear,  some  are  use- 
less, and  soon  disappear.  Others  are  of  some  value, 
and  will  be  preserved  by  natural  selection.  Now  it 
will  occasionally  happen  that  the  variations  may  be 
of  such  a  character  as  to  make  a  radical  change  in 
the  organism  and  fit  it  for  entirely  new  conditions. 
For  instance,  we  may  suppose  that  all  during  the 


134  THE  LIVING  WORLD. 

Triassic  (6),  Jurassic  (7),  and  Cretaceous  (8)  ages  the 
mammals  were  undergoing  constant  changes,  and 
were  subject  to  numerous  vicissitudes.  Nothing 
occurred,  however,  to  give  them  any  special  ad- 
vantage over  their  conditions  or  their  enemies,  and 
they  therefore  continued  for  this  long  period  with 
little  modification.  But  toward  the  end  of  the  Cre- 
taceous age  it  chanced  that  among  the  multitude  of 
variations  some  of  the  individuals  acquired  a  new 
character  in  regard  to  the  habit  of  reproduction. 
This  new  character  gave  them  the  power  of  retaining 
their  young  in  the  uterus  for  a  longer  time  than  before, 
carrying  them  indeed  until  they  were  well  developed. 
Now  we  know  from  the  study  of  animals  to-day  that 
this  character,  together  with  others  correlated  with  it, 
make  the  animal  thus  favored  very  much  more  pow- 
erful and  far  better  adapted  to  contend  in  the  strug- 
gle for  life.  We  may  not  be  able  to  say  why  it  is, 
but  it  is  certainly  true  that  the  mammals  with  this 
new  habit  in  reproduction  always  triumph  over  the 
marsupials.  It  is  easy  to  suppose,  therefore,  that 
as  soon  as  this  variation  occurred  in  the  mammalian 
stock  there  was  an  immediate  impulse  given  to  the 
development  of  the  group.  The  mammals  became 
more  active,  and  soon  proved  themselves  more  than 
a  match  for  the  large  reptiles.  In  the  struggle  for 
food  they  soon  triumphed  over  their  marsupial  pre- 
decessors, and  finding  thus  the  whole  world  open  to 
their  conquests,  they  multiplied  rapidly  and  gave 
rise  at  once  to  the  multiplicity  of  types  found  in  the 
early  Tertiary  (9).  A  new  field  had  thus  been  offered 
by  a  chance  (?)  anatomical  variation. 


A    VIEW  IN  PERSPECTIVE.  135 

Now  in  the  explanation  thus  given  we  must  of 
course  deal  somewhat  with  hypothesis.  We  do  not 
know  positively  that  it  was  the  occurrence  of  the 
new  variation  in  the  reproductive  habits  that  gave  the 
stimulus  to  the  mammals,  and  thus  caused  the  rapid 
divergence  in  the  Tertiary  period.  We  know,  how- 
ever, that  the  mammals  did  at  that  time  receive  a 
new  impulse  from  some  source,  and  did  immediately 
expand  to  fill  the  new  field  of  nature  open  to  them, 
and  we  have  pretty  good  evidence,  moreover,  that  at 
the  same  time  the  above-mentioned  change  in  the 
reproductive  organs  occurred.  From  all  of  this  we 
learn  to  picture  to  ourselves  the  animal  kingdom  as 
constantly  searching  after  new  fields  for  expansion. 
The  different  types  of  animals  are  constantly  migra- 
ting here  and  there,  constantly  subjected  to  new 
conditions,  and  therefore  constantly  undergoing 
change.  Generally  the  severe  competition  with 
nature  and  with  enemies  keeps  them  in  restraint 
by  cutting  off  all  sportive  branches,  and  allowing 
only  the  central  strongest  forms  to  continue  to  exist. 
But  occasionally  a  change  in  the  conditions  produced 
by  geological  forces  gives  certain  classes  the  advan- 
tage over  others,  or  changes  in  the  configuration  of 
the  land  produce  new  fields  for  expansion.  Or, 
again,  some  one  of  the  sportive  branches  proves 
to  have  inherent  qualities  of  great  vigor,  and  in  this 
way  offers  a  new  field  of  organic  type.  In  any  of 
these  cases  the  opportunity  is  seized 
ately,  and  shows  itself  in  rapid  expj 
tainly  a  fact  that  the  multipli< 
greatest  in  the  early  life  of  a  gei 


136  THE   LIVING  WORLD. 

As  already  stated,  it  is  very  seldom  that  we  can 
determine  what  were  the  circumstances  which  have 
produced  the  rapid  development  of  certain  types. 
We  may  suppose  that  the  marvellous  expansion  of 
the  reptiles  in  the  Mesozoic  was  due  to  the  fact  that 
these  were  the  first  true  air-breathing  vertebrates, 
and  had  therefore  the  whole  land  to  themselves. 
Such  an  unoccupied  field  would  undoubtedly  have 
had  a  tendency  to  stimulate  them  into  rapid  expan- 
sion. We  may  suppose  also  that  the  rapid  appear- 
ance of  the  numerous  order  of  insects  in  the  Jurassic 
(7)  and  Cretaceous  (8)  was  correlated  with,  if  not 
caused  by,  the  appearance  of  flowers  and  the  acquir- 
ing of  the  habit,  on  the  part  of  some  insects,  of 
feeding  upon  them.  So  too  the  rapid  expansion  of 
birds  in  the  Cretaceous  and  subsequent  age  was  due 
to  the  acquiring  of  aerial  powers,  which  powers  were 
of  course  due  to  internal  variations  in  the  direction 
of  the  production  of  wingsl  Perhaps  the  expansion 
of  mammals  was  due  to  the  change  in  reproductive 
habits  as  above  mentioned.  In  a  few  other  cases  it 
is  possible  to  make  a  guess  as  to  the  causes  which 
produced  expansion  of  certain  groups  of  animals, 
but  in  general  we  must  rest  satisfied  with  ignorance 
of  the  details,  and  with  only  the  general  explanation 
that  expansion  is  due  to  the  occupancy  of  a  new 
field  in  nature. 

A  General  Advance  from  the  Earliest  Ages. 

The  next  point  for  us  to  notice  is  that  there  has 
been  a  general  advance  in  the  animal  kingdom  from 
the  earliest  ages  until  now.  Taken  as  a  whole,  there 


A    VIEW  IN  PERSPECTIVE.  137 

is  no  doubt  that  there  has  been  an  increase  in  com- 
plexity and  diversity,  and  this  is  what  is  meant  by 
advance.  At  the  same  time,  when  we  attempt  to 
follow  this  law  into  particulars  it  is  by  no  means 
always  possible  to  do  so.  In  the  sub-kingdom  Verte- 
brata,  there  is  no  doubt  of  the  facts.  Beginning  in 
the  Silurian  (2)  with  the  lowest  form,  there  followed 
in  the  Devonian  (3)  multitudes  of  fishes ;  in  the  Car- 
boniferous (4)  came  the  next  higher  class,  the  Am- 
phibia, to  be  followed  in  the  next  era  by  the 
reptiles  ;  these  by  the  birds  in  the  Cretaceous  and 
the  mammals  in  the  Tertiary  (9),  and  lastly  by  the 
appearance  of  man  in  the  Quaternary  (10).  In  all 
this  we  see  a  continuous  advance.  In  the  Articulata, 
too,  it  is  possible  to  trace  the  same  advance.  Ap- 
pearing at  first  in  the  form  of  low  generalized 
trilobites  and  phyllocarides,  the  Crustacea  developed 
the  higher  orders  of  lobsters,  shrimps,  and  crabs  con- 
siderably later.  The  insects  seemed  to  appear  in  the 
Silurian  (2)  as  cockroaches,  which  belong  to  the 
lowest  order.  In  the  other  Paleozoic  ages  there 
appeared  indications  of  the  Neuroptera,  and  Coleop- 
tera,  and  Hemiptera,  which  are  always  recognized  as 
the  lowest  orders.  It  was  only  in  the  later  periods 
that  the  higher  orders  made  their  appearance. 

In  the  other  groups  of  the  animal  kingdom,  how- 
ever, it  is  difficult  to  recognize  any  striking  advance 
in  structure.  It  is  questionable  whether  we  can  say 
that  the  echinoderms  of  to-day  are  as  a  whole  of  a 
higher  type  than  those  of  the  Silurian,  and  a  like 
question  arises  with  respect  to  the  Ccelentera, 
Mollusca,  and  Brachiopoda.  At  the  same  time,  the 


138  THE  LIVING  WORLD. 

general  increase  in  complexity  even  in  the  case  of 
these  types  is  indicated  by  two  facts  at  least.  We 
see  that  in  early  periods  it  was  the  lower  orders  that 
were  most  abundant  and  widely  diversified,  while 
to-day  it  is  the  higher  forms  that  are  predominant. 
For  instance,  among  the  Articulata  it  was  the  low 
trilobites  and  cockroaches  that  existed  in  abundance 
and  diversified  profusion  in  early  times,  while  to-day 
it  is  the  higher  orders  that  are  the  most  abundant, 
even  though  in  the  insects  the  lower  orders  are  still 
in  existence.  So  in  the  case  of  the  echinoderms,  the 
most  abundant  order  of  early  ages  was  the  Crinoidea 
although  the  other  classes  of  echinoderms  were  all  in 
existence.  To-day  though  the  crinoids  are  still  in 
existence,  it  is  the  higher  orders  that  are  the  most 
abundant. 

The  second  fact  indicating  the  general  advance  is 
the  greater  diversity  of  life  to-day  than  in  the  earliest 
times.  Some  groups  have  indeed  passed  into  de- 
cline or  disappeared,  but  most  of  those  that  do  exist 
are  to-day  in  greater  profusion  than  in  any  of  the 
past  ages.  Taken  as  a  whole,  it  is  one  of  the  most 
evident  teachings  of  the  history  of  life  that  there 
has  been  through  all  the  ages  a  constant  increase 
in  the  profusion  of  living  things,  and  a  continually 
growing  diversity  of  form.  Even  though  it  is  possi- 
ible  to  say  that  many  of  the  families  are  really  no 
more  highly  developed  than  were  their  represen- 
tatives in  the  past,  still  the  fact  of  the  increase  of 
diversity  of  type  is  a  plain  indication  of  a  general 
advance.  Thus  the  fact  of  the  evident  advance  in  the 
structure  of  the  representatives  of  some  types,  the 


A    VIEW  IN  PERSPECTIVE.  139 

increase  in  the  profusion  of  the  higher  orders  of  all 
classes,  and  the  general  increase  in  the  abundance 
and  diversity  of  the  families  of  animals  which  we  can 
trace  through  all  ages,  shows  conclusively  that  there 
has  been  a  distinct  advance  in  complexity  ef  type 
from  the  earliest  times  up  to  to-day. 

Although  there  has  been  thus  a  general  advance, 
it  is  certainly  true  that  it  has  not  affected  all  orders 
of  animals  alike.  We  have  already  noticed  that  some 
have  remained  practically  stationary  in  type.  Some 
have  actually  gone  down  hill  instead  of  up.  Many 
groups  of  animals  have  advanced  to  a  culmination 
and  then  begun  to  disappear  ;  and  in  the  disappear- 
ing of  animals  we  can  frequently  discern  evidence  of 
a  reversal  of  the  process  of  development,  the  later 
appearing  animals  becoming  of  a  distinctly  lower 
type  than  their  relatives  at  the  period  of  culmination. 
This  has  been  specially  well  shown  in  the  ammon- 
ites which  passed  out  of  existence  in  the  Cretaceous 
(8).  In  many  orders  of  animals  the  degeneration  has 
been  produced  from  other  causes.  Degeneration  is 
always  certain  in  the  animal  kingdom  when  any 
organ  ceases  to  be  used.  Many  orders  of  animals  by 
becoming  parasitic  upon  others  cease  to  have  any 
use  for  some  of  their  organs.  The  result  is  a  very 
general  degeneration  until  the  structure  of  the  ani- 
mal as  a  whole  has  become  markedly  degraded. 
Any  of  the  orders  of  parasitic  animals  will  serve  as 
illustrations  of  this  fact,  and  all  of  the  sub-kingdoms 
show  examples  of  such  degradation.  Among  fami- 
liar examples  may  be  mentioned  fleas,  which  have 
lost  their  wings ;  tape  worms,  which  have  lost  all  of 


140 


THE   LIVING  WORLD. 


their  digestive  organs,  etc.  Even  the  barnacles  may 
be  included  under  this  head,  for  although  not  para- 
sitic, they  have  acquired  stationary  habits,  and  have 
lost  the  locomotive  organs  they  once  possessed. 
While,  then,  the  general  history  of  the  animal  king- 
dom has  been  an  advance,  this  advance  is  compatible 
with  many  cases  of  retrogression.  All  groups  ad- 
vance to  a  culmination,  and  then  decline  through 
decaying  forms,  and  innumerable  instances  of  para- 
sitism have  produced  thus  their  inevitable  degrading 
effects. 


CHAPTER  VI. 

A   VIEW    IN    PERSPECTIVE. — (CONTINUED.) 

Soft-Bodied  A  nimals. 

LOOKING  at  the  animal  kingdom  as  a  whole,  we 
are  now  in  position  to  trace  something  like  a  general 
outline  of  its  history.  Life  appeared,  very  probably, 
before  the  land  was  fit  for  habitation,  and  conse- 
quently all  animals  were  marine ;  at  all  events,  it  is 
certain  that  the  earliest  animals  lived  in  the  ocean. 
Now,  drawing  our  conclusions  from  the  study  of 
embryology  and  comparative  anatomy,  we  learn  that 
the  earliest  animals  were  soft-bodied,  and  were  pro- 
vided with  no  hard  parts  for  defence  or  protection. 
Their  method  of  defence  was  probably  the  same  as 
that  of  the  low  soft-bodied  animals  of  to-day.  The 
smaller  forms  were  endowed  with  marvellous  insensi- 
bility to  injury,  and  with  great  powers  of  multiplica- 
tion. Cutting  them  to  pieces  simply  caused  their 
multiplication,  and  this  power  of  resisting  injury 
served  in  the  place  of  protective  organs.  The  larger 
forms,  like  Ccelentera,  were  provided  with  stinging 
hairs  for  defence,  and  they  also  possessed  remarkable 
powers  of  recovery  from  injury.  Having  no  sup- 
porting organs,  they  were  never  large  animals,  but 
they  doubtless  filled  the  early  seas  in  abundance. 

141 


142  THE  LIVING  WORLD. 

Development  of  Hard  Parts. 

The  next  step  in  the  history  was  the  development 
of  hard  parts  for  support  and  protection.  This  for 
the  first  time  made  the  existence  of  large  animals  a 
possibility.  Almost  immediately  after  the  diver- 
gence of  types  of  animals,  the  development  of  some 
form  of  skeleton  began.  The  echinoderms  developed 
plates  of  lime  in  their  skin  ;  the  mollusks  secreted 
upon  their  bodies  a  thick  shell  of  the  same  material ; 
some  of  the  hydroids  developed  an  external  shell, 
and  the  corals  a  calcareous  framework ;  the  Articu- 
lata  (except  the  so-called  worms)  developed  upon  the 
outside  of  their  body  a  light  but  tough  shell  of 
chitin.  Such  skeletons  as  these,  while  they  were 
certainly  of  great  value,  were  in  one  respect  a  disad- 
vantage. In  the  mollusks,  for  instance,  they  were 
large  and  clumsy  compared  with  the  size  of  the 
animal ;  they  prevented  rapid  motion  and  effectually 
checked  any  great  increase  in  size.  It  was  only  in 
later  times,  when  the  mollusks  got  rid  of  their  clumsy 
shell,  that  any  very  great  size  could  be  attained,  as 
in  the  case  of  the  great  giant  squids.  In  the  Articu- 
lata  the  shell,  made  of  chitin  instead  of  lime,  was 
lighter  and  tougher,  and  therefore  better  adapted  to 
activity.  As  a  result,  the  Articulata  succeeded  in 
producing  such  active  and  well-protected  groups  as 
the  insects.  But  the  external  shell  of  insects  and 
crustaceans  is  not  fitted  for  any  large  animal,  the  old 
trilobites  (some  of  which  were  over  two  feet  in 
length)  reaching  the  extreme  in  this  direction. 

A  little  later  another  division  of  animals  hit  upon 
a  new  device  for  the  support  and  protection  of  its 


A     VIEW  IN  PERSPECTIVE.  143 

soft  parts.  An  internal  skeleton  was  developed. 
This  first  appeared  as  a  soft  but  resisting  rod  (noto- 
chord),  inside  of  the  back,  and  reaching  from  head  to 
tail.  This  rod  gave  some  strength,  but  could  not 
become  resisting  without  destroying  the  flexibility 
of  the  body.  Therefore  it  soon  became  broken  into 
segments  to  give  the  body  flexibility,  and  then  it 
hardened  into  a  series  of  bones  and  formed  the  spinal 
column  and  the  basis  of  an  internal  skeleton.  Of 
this  early  history  we  have  no  trace  from  fossils,  for 
it  was  not  until  this  rod  had  hardened  into  cartilage 
that  it  was  possible  for  vertebrates  to  be  preserved 
in  the  rocks.  The  early  history  of  the  vertebrates, 
therefore,  must  be  read  from  embryological  evidence. 
This  type  of  an  internal  skeleton  immediately  proved 
to  be  a  success.  This  means  of  support  gave  strength 
and  rigidity  to  the  soft  parts,  and  at  the  same  time 
did  not  burden  them  so  much  as  to  render  them 
unwieldy,  for  it  was  adapted  on  mathematical  prin- 
ciples for  furnishing  the  greatest  strength  with  the 
least  bulk. 

The  value  of  this  skeleton  immediately  showed 
itself  by  the  increase  in  size  of  the  animals  possess- 
ing it.  We  find  that  there  followed  now  a  new 
phase  of  animal  history.  For  a  long  series  of  succeed- 
ing ages  we  see  an  increase  in  size  among  the  higher 
members  of  the  animal  kingdom.  Doubtless  these 
early  vertebrates  had  fierce  contests  with  other  ani- 
mals, but  the  large  and  active  vertebrates  soon 
proved  themselves  superior  to  the  clumsy  mollusks 
and  the  sluggish  trilobites.  The  struggle  of  verte- 
brates with  each  other  was  severe.  Size  was  a  factor 


144  THE   LIVING  WORLD. 

of  the  utmost  importance  in  these  contests,  and 
through  all  the  history  of  the  vertebrates  we  find 
that  the  development  of  each  class  was  marked 
by  an  increase  in  size.  This  tendency  toward  in- 
crease in  size  continued  until  the  Jurassic  (7),  when 
it  reached  its  culmination  in  the  huge  reptiles  of 
this  period,  some  of  which  were  the  largest  animals 
that  ever  lived.  Of  course,  we  do  not  mean  that 
every  order  of  vertebrates  became  of  large  size,  for 
many  of  the  smaller  ones  did  establish  their  right  to 
live.  Many  small  fishes  and  small  reptiles  did  suc- 
ceed in  perpetuating  themselves.  Nevertheless  we 
can  recognize  the  truth  of  the  general  fact  that  from 
the  Devonian  (3)  to  the  Cretaceous  (8)  or  Tertiary 
(9)  a  tendency  towards  increase  in  size  marked  the 
history  of  the  animal  kingdom. 

This  increase  in  size,  however,  was  sure  to  reach 
a  limit.  Great  size  is  only  to  be  possessed  at  the 
expense  of  activity  and  agility,  and  the  great  reptiles 
of  the  Jurassic  probably  became  so  large  that  it  was 
a  matter  of  greater  and  greater  difficulty  for  them  to 
procure  sufficient  food.  As  we  have  already  seen, 
from  the  Tertiary  period  the  mammals  began  to 
supersede  the  reptiles  as  the  monarchs  of  creation. 
Even  among  the  h'ighest  class  of  animals  we  find  for 
a  long  time  a  tendency  to  produce  animals  of  great 
size.  Some  of  the  mammal  orders  of  the  Tertiary 
(9)  attained  a  size  which  almost  made  them  rivals  in 
bulk  with  the  reptiles.  But  we  soon  notice  in  the 
history  of  this  class  a  tendency  to  develop  small 
size  and  agility  instead  of  huge.  bulk.  Of  the  eden- 
tates, the  huge  Megatherium  disappeared,  and  only 


A    VIEW  IN  PERSPECTIVE.  145 

the  small  animals,  such  as  armadillos  and  sloths 
succeeded  in  perpetuating  themselves.  The  gigantic 
Dinotherium  and  the  Mastodon  were  exterminated, 
and  only  two  species  of  elephants  remain  to  repre- 
sent this  once  abundant  type.  In  general,  among 
mammals,  it  was  the  smaller  animals,  those  which 
developed  speed,  as  in  the  ungulates,  or  immense 
activity  and  strength  in  capturing  prey,  as  in  the 
Carnivora,  that  gradually  became  the  most  abundant, 
and  thus  eventually  the  predominant  types.  The 
development  of  bulk  had  been  superseded  by  a  new 
phase  of  progression.  Increased  activity,  as  shown 
by  the  power  of  flight,  and  the  development  of  claws 
and  teeth  for  defence,  took  the  place  of  size  as  the 
most  potent  factor  in  the  struggle  for  existence. 

The  Mental  Factor. 

But  this  phase  of  nature  was  soon  to  yield  to 
another  higher  force.  From  the  earliest  record  we 
have  of  animals  we  can  see  an  indication  of  the  final 
era  in  the  history,  i.  e.,  the  era  of  mental  activity. 
The  brain  of  the  lower  vertebrates  was  small  and 
indicated  the  possession  of  little  intelligence.  The 
same  was  true  of  the  brain  of  early  mammals.  In 
some  of  them  the  brain  was  so  small  that  it  could 
be  passed  through  the  neural  canal  of  the  lumbar 
vertebrae,  and  was  thus  only  a  little  larger  than  the 
spinal  cord.  From  the  early  Tertiary  (9)  period, 
however,  the  size  of  the  brain  has  been  increasing. 
At  the  same  time  with  its  increase  in  size,  the  brain 
has  been  increasing  also  in  complexity,  the  cerebral 
lobes  becoming  larger  and  showing  more  of  a  ten- 

10 


146  THE  LIVING    WORLD. 

dency  toward  convolution.  There  is  hardly  a  more 
significant  fact  in  the  history  of  animals,  when  we 
remember  the  approaching  advent  of  man,  than  this 
increase  in  the  size  of  the  brain  of  mammals  from 
the  early  Tertiary  (9).  Undoubtedly  this  increase 
in  the  size  of  the  brain  was  connected  with  the  in- 
creased activity  of  the  mammals  already  mentioned, 
for  activity  and  agility  imply  delicate  control  of  the 
lower  motor  centres  by  the  higher  centres  of  the 
brain,  and  this  requires  more  brain  power.  Active 
animals  always  have  relatively  large  brains.  But 
undoubtedly  also  the  increase  in  the  size  of  the 
brain  was  accompanied  by  an  increasing  amount  of 
intelligence.  In  short,  with  the  development  of  the 
mammals  there  had  appeared  a  time  when  bulk  had 
given  place  to  muscular  activity  and  agility,  but  at 
the  same  time  the  growth  of  the  brain  was  gradually 
preparing  the  way  for  the  time  when  intelligence 
should  take  the  place  of  brute  force. 

« 
Instinct. 

Already,  in  an  entirely  different  line  of  descent,  do 
we  find  the  mental  nature  becoming  predominant. 
The  higher  order  of  insects  has  as  its  chief 
character  the  habit  of  living  in  colonies,  and  the 
consequent  development  of  remarkable  instincts. 
As  a  rule,  the  insects  seem  to  have  depended  upon 
their  powers  of  rapid  multiplication  as  their  chief 
means  of  defence.  Most  insects  are  weak  animals, 
and  all  are  small,  but  their  immense  powers  of  re- 
production safely  defend  them  from  extermination. 
Some  of  the  higher  orders,  however,  learned  to  band 


A    VIEW  IN  PERSPECTIVE.  147 

themselves  together  into  colonies,  and  then  devel- 
oped a  most  complicated  set  of  instincts,  mostly 
adapted  for  the  preservation  and  integrity  of  the 
colony.  Now  while  instinct  is  indeed  a  mental 
factor  quite  distinct  from  intelligence,  it  is,  like  in- 
telligence, a  function  of  the  nervous  system.  The 
insects  thus  first  inaugurated  the  development  of 
the  nervous  system  to  an  extent  which  made  its 
powers  prominent  factors  in  the  preservation  of  the 
race.  At  what  geological  date  instinct  became  such 
a  prominent  factor  in  the  life  of  insects  we  cannot 
say.  The  individuals  found  in  the  Cretaceous  (8) 
were  all  sexual  individuals.  Now  the  formation 
of  colonies  is  usually  accompanied  by  the  produc- 
tion of  sexless  individuals  (neuters,  workers).  It 
would  therefore  seem  that  the  insects  of  the  Creta- 
ceous had  not  yet  acquired  their  social  habits,  and 
that  the  high  development  of  instinct  was  subsequent 
to  this  period.  At  all  events  it  was  a  late  event  in 
the  history  of  insects,  just  as  the  development  of 
intelligence  was  a  late  event  in  the  life  of  verte- 
brates, and  it  seems  probable  that  instinct  reached  a 
high  development  in  the  Tertiary  (9)  before  the 
special  development  of  intelligence  began. 

Intelligence. 

We  are  now  prepared  to  recognize  a  period  in 
the  history  of  animals  when  intelligence  had  be- 
come the  predominant  feature  of  nature.  This  of 
course  brings  us  to  man,  and  we  are  now  in  posi- 
tion to  consider  the  important  question  of  the 
relation  of  man  to  this  history  of  life.  For  our  pur- 


148  THE   LIVING  WORLD. 

pose  there  is  no  need  of  deciding  whether  man  is 
a  final  step  in  the  line  of  evolution  or  is  to  be 
regarded  in  some  measure  as  a  special  creation. 
Man,  by  virtue  of  the  powers  he  possesses,  holds  an 
important  position  in  this  life's  history,  and  his  posi- 
tion is  the  same  whether  his  unique  powers  be 
regarded  as  evolved  from  those  of  animals  or  as 
newly  created.  The  hint  has  already  been  dropped 
that  the  essential  feature  of  man  is  the  importance 
of  his  mental  nature,  and  if  we  add  to  this  his  ethi- 
cal nature,  we  have  included  all  features  of  distinct 
importance  that  belong  to  him.  Man  may,  indeed, 
be  not  incorrectly  defined  as  the  animal  in  which 
everything  has  become  subordinate  to  the  nervous 
system.  For  his  existence  in  the  world  he  depends 
not  upon  his  physical  force  nor  anatomical  structure. 
He  proves  his  right  to  live  neither  by  defensive  nor 
offensive  armor,  neither  by  superior  strength  nor 
superior  powers  of  multiplication.  The  possession 
of  mind  alone  is  his  pride.  No  sooner  did  his  men- 
tal nature  become  a  predominant  feature  than  his 
physical  nature  ceased  to  undergo  any  considerable 
development.  It  is  one  of  the  curious  facts  of  na- 
ture that  man,  who  possesses  powers  of  mind  so 
infinitely  superior  to  those  of  the  highest  animal 
that  there  is  really  no  comparison  between  them, 
should  at  the  same  time  be  so  much  like  the 
apes  in  his  anatomical  structure  that  naturalists 
think  he  ought  to  be  classed  in  the  same  genus  with 
them.  Anatomically,  then,  man  is  an  ape,  while,  so 
far  as  mental  powers  are  concerned,  he  deserves  a 
new  kingdom  to  himself. 


A    VIEW  IN  PERSPECTIVE.  149 

It  is  not  difficult  to  find  an  explanation  for  this 
surprising  fact.  Among  animals  physical  power  is 
usually  the  only  feature  by  which  one  animal  con- 
quers another.  With  them,  therefore,  when  a  new 
species  is  established  it  must  be  for  some  superiority 
in  physical  ability.  As  a  result  the  anatomical  struc- 
ture of  animals  changes  with  every  advance,  for 
each  new  species  must  be  in  some  respects  better 
adapted  to  its  conditions  than  the  older  one  from 
which  it  came.  With  man,  however,  the  physical 
side  of  nature  is  comparatively  of  no  importance. 
From  the  moment  that  he  proved  himself  master  of 
animals  by  his  intelligence,  the  development  of  his 
physical  nature  became  a  very  subordinate  matter. 
He  made  artificial  weapons  for  defence  and  offence 
far  superior  to  any  that  could  be  supplied  him  by 
nature,  and  by  intelligent  use  of  them  he  became 
far  more  efficient  than  any  amount  of  muscular 
energy  could  have  made  him.  Now  nature  never 
supplies  animals  with  organs  for  which  they  have  no 
use,  and  therefore  the  development  of  man's  body 
practically  ceased  with  the  beginning  of  the  develop- 
ment of  his  mind.  His  hand,  it  is  true,  has  become 
more  delicate,  since  that  is  an  organ  of  great  use  to 
his  intelligence.  Some  other  changes,  perhaps,  also 
appeared,  but  except  in  a  few  superficial  features 
man  remained  in  anatomy  essentially  as  he  was  at 
the  outset,  while  the  growth  of  his  mental  powers 
soon  produced  between  him  and  the  animal  king- 
dom a  wide  chasm  which  we  cannot  bridge  even  in 
imagination. 

Here,  then,  we  readily  see  why  it  is  that  there  has 


ISO  THE  LIVING  WORLD. 

been  so  much  confusion  and  disagreement  in  the 
attempts  to  place  man  in  the  scheme  of  classifica- 
tion. Relying  upon  his  bodily  structure,  he  is  un- 
doubtedly to  be  placed  with  the  higher  primates, 
and  he  is  therefore  ranked  with  the  apes  in  all 
schemes  of  natural  classifications.  But  he  differs 
from  all  other  animals  in  having  as  his  essential 
character  the  development  of  a  new  side  of  his 
nature  which  is  not  primarily  anatomical.  When  in 
the  pre-Tertiary  times  certain  of  the  vertebrates 
acquired  a  new  character  connected  with  the  repro- 
ductive system,  there  soon  arose  a  type  of  animals 
which  we  call  mammals.  Now  classification  is  al- 
ways based  on  structure,  and  we  therefore  call  this 
new  group  of  animals  a  new  class,  because  of  their 
new  anatomical  character  and  from  the  fact  that 
connected  with  it  were  other  changes  in  anatomy 
which  radically  modified  the  type  of  animal.  When, 
however,  the  final  race  acquired  his  new  character  of 
mental  development,  we  do  not  regard  the  resulting 
animal  as  a  new  class,  for  in  this  case  the  new  de- 
parture did  not  involve  any  great  changes  in 
anatomical  type.  We  can  therefore  readily  sympa- 
thize with  both  classes  of  naturalists,  one  of  which 
regards  man  as  a  species  of  the  ape  family,  while 
the  other  recognizes  three  kingdoms — animals, 
plant,  and  man.  Either  extreme  of  classification  is 
perhaps  more  justified  than  an  intermediate  position. 

Divergence  of  Character  Marks  the  Development  of 
Animals. 

With  the  appearance  of  man  there  is  the  begin- 
ning of  a  new  law  in  nature,  and  one  of  great  sig- 


A    VIEW  IN  PERSPECTIVE.  15 1 

nificance.  The  development  of  mind,  and  especially 
of  the  ethical  *  nature  of  man,  is  producing  a  result 
in  the  human  race  which  in  a  measure  reverses  the 
results  of  the  law  of  natural  selection,  and  the  other 
laws  governing  animals. 

As  we  have  seen,  the  history  of  the  animal  king- 
dom is  such  as  can  be  best  explained  in  the  form  of 
a  branching  tree.  In  accordance  with  the  laws  of 
nature,  the  most  important  of  which  is  the  law  of 
natural  selection,  the  descendants  of  any  line  of 
animals  gradually  diverge  from  each  other  like  the 
branches  of  a  tree.  For  example  the  descendants  of 
the  early  type  of  mammals  gradually  assumed  dif- 
ferent characters  along  different  lines  until  there 
was  produced  the  abundance  of  mammal  orders 
found  in  the  early  Tertiary  (9).  The  exact  way  in 
which  the  laws  of  nature  work  to  produce  such  a 
divergence  is  a  matter  under  discussion  to-day  by 
naturalists.  Darwin  tried  to  show  that  his  law  of 
natural  selection  was  in  itself  sufficient  to  explain 
this  divergence.  Further  study  makes  this  more 
doubtful,  or  at  all  events  requires  the  addition  of 
certain  other  factors  favoring  the  isolation  of  indi- 
viduals. But  whatever  be  the  difference  in  our  ideas 
of  the  details,  there  is  no  question  that  the  laws  of 
nature  under  which  animals  live  and  multiply,  result 
in  the  production  of  what  is  known  as  divergence  of 
character. 

Now  the  essential  feature  of  a  divergence  of  char- 
acter is  isolation  and  separation.  In  some  way  the 

*  By  the  development  of  the  ethical  nature  we  would  not  mean  to 
imply  that  conscience  has  been  evolved  from  the  other  attributes  of 
man,  but  simply  that  since  the  appearance  of  man  the  ethical  element 
has  developed  to  a  higher  grade. 


152  THE  LIVING  WORLD. 

descendants  of  one  pair  of  animals  become  asso- 
ciated into  groups.  Each  group  has  characters  of 
its  own,  and  since  it  remains  isolated  from  the 
others,  at  least  so  far  as  interbreeding  is  concerned, 
it  soon  establishes  for  itself  a  distinct  line  of  descent, 
a  new  race  or  a  new  species.  Later  in  the  history 
perhaps  its  own  descendants  in  like  manner  become 
separated  into  still  other  groups,  and  so  on,  the 
divergence  becoming  wider  and  wider  as  the  cen- 
turies roll  by.  Without  some  sort  of  isolation  and 
separation  into  groups  divergence  of  character  is 
impossible. 

Convergence  of  Character  the  Result  of  Human 
Development. 

The  mind  and  ethical  nature  of  man,  and  especially 
the  law  of  Christ,  under  which  man  is  slowly  learn- 
ing to  live,  is  producing  a  slow  but  sure  modification 
of  the  history  of  development.  Instead  of  producing 
isolation  aud  divergence,  they  are  producing  union  and 
convergence. 

Man  is  distinctly  a  social  animal.  Among  the 
lower  animals  there  are  some  that  have  what  are 
known  as  social  instincts.  Instead  of  living  as 
isolated  individuals  they  associate  in  groups  for 
mutual  protection.  Among  animals  of  low  intel- 
ligence, like  fishes,  this  has  little  significance  in  the 
development.  With  higher  animals,  however,  social 
habits  produce  more  effect.  With  the  higher  insects, 
these  social  habits  have  been  developed  to  their 
highest  point.  The  marvellous  series  of  instincts  so 
well  known  among  ants  and  bees  are  unquestionably 


A    VIE  W  IN  PERSPECTIVE.  153 

the  results  of  these  social  habits.  But  even  in  insects 
the  social  instincts  are  very  narrow  in  their  applica- 
tion, for  although  they  produce  association  of  indi- 
viduals into  colonies,  although  they  are  frequently 
so  far  developed  that  the  individual  sacrifices  his  life 
for  the  good  of  the  colony,  they  do  not  give  rise  to 
anything  like  an  association  of  the  colonies  with  each 
other.  Each  colony  of  insects  still  keeps  up  the 
natural  warfare  with  other  colonies.* 

Man  is  also  universally  a  social  animal,  and  during 
all  his  history  has  lived  in  communities.  In  early 
history,  as  among  savages  to-day,  men  united  for 
mutual  protection  into  communities  or  tribes.  But 
while  the  members  of  each  tribe  show  friendship  for 
each  other,  the  separate  tribes  have  retained  the 
same  sort  of  mutual  enmity  that  is  possessed  by  the 
colonies  of  social  animals.  Hostility  is  the  constant 
relation  of  the  tribes  to  each  other,  a  hostility  per- 
haps even  more  constant  than  among  animals.  This 
constant  hostility  produced  a  more  or  less  complete 
separation  of  the  tribes  from  each  other,  and  the  re- 
sult, as  in  the  rest  of  the  animal  world,  was  a  gradual 
separation  of  the  tribes  from  each  other.  These  are 
exactly  the  conditions  necessary  for  the  production 
of  divergence  in  character  and  the  formation  of  new 
races.  To  this  extent  then  the  development  of  the 
races  of  men  was  similar  to  the  development  of  races 
of  animals. 

But  almost  from  the  beginning  of  the  development 
of  man  we  notice  a  new  law  dependent  on  the  pos- 
session of  conscience  and  the  feeling  of  love.  The 

*  To  this  statement  there  are  a  few  exceptions. 


154  THE  LIVING  WORLD. 

ethical  element  of  man's  nature,  though  very  rudi- 
mentary in  early  history,  has  always  belonged  to 
man.  Its  basis  is  love.  Now  love  is  really  a  new 
feeling  in  nature,  at  least  man  is  the  first  animal  in 
which  it  becomes  of  importance  enough  to  influence 
his  development.  Among  the  higher  animals,  such 
as  birds  and  mammals,  we  do  find  a  maternal  love  of 
the  mother  for  her  offspring,  but  the  love  ceases  with 
the  maturity  of  the  young.  Even  among  social 
animals  there  is  found  nothing  that  corresponds  to 
the  love  of  one  individual  for  another  in  its  broader 
sense.  Animals  combine  for  mutual  protection-  but 
seem  perfectly  indifferent  to  each  other  except  in  so 
far  as  the  advantage  of  the  whole  community  is  con- 
cerned. A  soldier  ant  may  sacrifice  his  life  for  his 
colony,  but  he  shows  no  feeling  for  another  indi- 
vidual in  distress.  We  would  not  deny  that  occa- 
sionally there  are,  even  among  animals,  slight  traces 
of  what  must  be  regarded  as  the  feeling  of  love  in 
a  higher  sense.  It  is  certain,  however,  that  such  in- 
stances are  few,  and  that  the  feeling  of  love  is  not 
one  which  can  be  regarded  as  having  any  considera- 
ble influence  upon  the  development  of  the  animals. 
With  man,  however,  intelligence  and  conscience 
have  produced  different  conditions.  Even  in  the 
savage  tribe  a  feeling  of  love  for  one's  relatives  or 
friends,  patriotism  for  one's  tribe,  self-sacrifice  for 
the  good  of  the  community,  are  characters  which 
receive  the  highest  honor.  While  then  with  man,  as 
with  animals,  the  tribal  relations  may  be  primarily 
for  mutual  protection,  it  is  certain  that  the  feeling 
of  love  for  one  another,  the  ethical  element  of  human 


A    VIEW  IN  PERSPECTIVE.  155 

nature,  has  had  at  all  times  the  important  effect  of 
cementing  the  tribes  into  a  rigid  unity.  Union  for 
mutual  protection  is  impossible  without  it.  Among 
the  Jews  we  early  find  the  command  to  love  one's 
neighbor  as  one's  self.  Their  interpretation  of  the 
word  neighbor  was  to  be  sure  very  narrow,  but  the 
presence  of  such  a  law  shows  that  love  was  recog- 
nized as  one  of  the  noblest  attributes  of  man. 

The  tribal  relation  was  thus  originally  assumed  for 
protection,  but  a  mutual  love  cemented  the  tribes 
into  units.  By  further  application  of  the  same  feel- 
ings, the  size  of  the  tribes  increased,  and  they  were 
finally  submerged  into  nations.  Now  it  is  the  feel- 
ing of  love  which  alone  makes  great  nations  possible. 
The  increase  in  the  size  of  tribes  and  nations  has 
undoubtedly  been  brought  about  by  conquest  rather 
than  by  love.  But  it  is  the  feeling  of  love  alone 
which  enables  a  large  tribe  to  hold  together.  The 
tribe  whose  sympathies  were  confined  within  narrow 
limits  would  always  disappear  before  the  tribe  whose 
broader  love  made  possible  the  union  of  larger 
bodies.  Those  tribes  in  which  this  feeling  of  mutual 
love  and  sympathy  was  the  broadest  obtained  the 
mastery  over  the  others  and  increased  at  the  ex- 
pense of  the  others.  Now  since  all  tribes  and 
nations  have  recognized  a  demand  on  man  to  love 
other  members  of  his  own  nation,  the  scope  of  man's 
obligation  to  love  his  fellow  men  has  constantly  ex- 
panded with  the  increase  in  the  size  of  nations. 
Finally,  with  Christ  there  was  announced  the  com- 
plete law  for  man,  the  law  of  universal  love.  By 
Christ  was  man's  obligation  to  love  extended  to  his 


156  THE  LIVING  WORLD. 

enemies,  by  him  was  the  word  neighbor  defined  so 
as  to  include  every  one  who  needed  help,  whether  of 
the  same  tribe  or  nation  or  another,  whether  friend 
or  foe.  Thus  it  was  that  love,  the  special  attribute 
of  man,  was  so  broadened  as  to  include  all  mankind 
and  to  bring  about  a  universal  brotherhood.  This 
law  of  Christ  looks  towards  the  destruction  of  the 
tribal  relation,  and  the  national  relation  as  well,  and 
when  it  is  fully  established  as  the  law  of  man,  it  will 
produce  one  nation,  one  association,  which  shall 
combine  all  of  mankind  into  one  union  of  mutual 
assistance  and  love.  We  are  far  enough  from  such  a 
condition  at  present.  It  is  the  millenium  of  which 
we  sometimes  dream,  and  toward  which  our  progress 
seems  slow  enough.  But  the  development  of  man 
is  tending  in  this  direction,  now  that  he  has  once 
recognized  that  universal  love  is  the  law  of  his  life. 

Looking  forward  then  into  the  coming  centuries 
we  see  the  vegetable  world  remaining  practically  as 
it  is,  except  as  it  is  modified  by  the  interference  of 
man  in  exterminating  plants  not  of  value  to  him, 
and  improving  those  which  he  uses.  We  see  the 
animal  kingdom  on  the  land  largely  exterminated  by 
the  power  of  man,  though  life  in  the  ocean  may  for 
a  long  time  remain  unchanged.  But  the  marine 
types  offer  no  chance  for  the  future,  for  they  are  all 
low  ones  which  reached  their  culmination  in  the  past 
ages.  But  we  see  mankind  left,  the  creature  of  God, 
advancing  in  intelligence,  knowledge,  and  morality, 
to  the  end  which  we  do  not  see  and  cannot  imagine. 
Whether  there  be  a  phase  in  our  nature  superior  to 
mind,  which  shall  in  the  future  ages  be  brought  into 


A    VIEW  IN  PERSPECTIVE.  I  57 

expansion  and  produce  a  new  race  of  beings  we  can- 
not tell,  but  the  era  of  animal  life  ended  when  that 
of  man  began. 

It  was  not  to  be  expected,  of  course,  that  this  law 
as  announced  by  Christ  would  be  at  first  understood. 
The  human  race  in  his  day  had  hardly  entered  into 
the  conception  of  the  beauty  of  love  in  its  narrower 
sense  of  love  to  one's  neighbor,  and  it  was  certainly 
not  ready  to  accept  the  definition  of  neighbor  as  in- 
cluding one's  enemies.  It  is  not  our  purpose  here  to 
enter  into  a  consideration  of  the  history  of  the  slow 
growth  of  this  new  law.  Even  yet  we  fail  to  accept 
it,  for  the  successful  general  receives  our  highest 
honors,  and  the  soldier  is  our  greatest  hero.  We  even 
fail  to  understand  the  law.  The  larger  our  nations 
grow,  the  more  comprehensive  become  our  obliga- 
tions, but  we  do  not  yet  believe  that  the  time  will 
come  when  the  Chinaman,  the  African,  and  the  Cauca- 
sian will  actually  unite  into  a  common  brotherhood. 

All  this  lies  beyond  our  present  purpose.  The 
relation  of  the  new  law  to  the  history  of  life,  how- 
ever, is  very  important  for  us  to  understand.  This 
law,  as  soon  as  it  is  applied  to  human  life,  produces 
no  longer  divergence  of  character  but  convergence. 
We  have  seen  that  the  essence  of  divergence  is  sep- 
aration and  isolation  of  groups  of  individuals.  The 
races  of  mankind  which  we  find  to-day  have  been 
produced  by  such  a  lack  of  intercourse  among  the 
tribes  of  men  and  by  the  constant  enmity  and  war- 
fare of  early  nations  which  have  tended  to  keep  up  a 
constant  isolation.  But  with  the  broadening  of 
man's  application  of  his  obligation  to  love  his  neigh- 


158  THE  LIVING  WORLD. 

bor,  the  nations  become  larger,  and  the  increase  in 
the  size  of  the  nations  acts  against  the  increase  in 
their  diversity.  As  fast  as  the  members  of  a  nation 
become  cemented  together,  so  fast  do  they  begin  to 
assume  common  characters.  The  American  nation 
is  absorbing  into  itself  a  great  variety  of  people, 
perhaps  faster  than  it  can  assimilate  them.  But  as 
fast  as  they  are  absorbed,  just  so  fast  does  the  char- 
acter of  the  nation  change.  The  American  nation 
no  longer  possesses  the  character  of  the  men  who 
struggled  for  independence  a  century  ago.  The 
history  of  civilization  has  been  always  marked  by 
the  absorption  of  the  small  races  into  the  larger 
ones.  It  is  only  enmity  and  a  narrow  patriotic  love 
that  prevents  all  of  the  smaller  nations  of  Europe 
from  becoming  parts  of  the  larger  ones.  Thus  it  is 
that  to-day  the  formation  of  new  races  has  been 
checked,  at  least  among  the  higher  classes  of  man- 
kind. The  nations  are  growing  larger,  their  hos- 
tilities are  becoming  lessened,  intercourse  of  com- 
merce and  friendship  between  them  is  increasing, 
and  with  all  this  a  tendency  toward  unification  and 
concentration  is  plainly  seen.  Not  divergence  but 
convergence  of  type  is  the  history  of  to-day. 

It  is  therefore  plain  that  the  ethical  nature  of  man 
is  producing  a  new  phase  in  the  development  of  the 
world.  It  is  checking  the  tendency  to  formation  of 
new  types,  and  is  tending  to  unite  into  one  the 
members  of  the  race.  To-day  intelligence  is  uniting 
all  men  into  closer  and  closer  relations  of  commerce 
and  education.  In  the  future  we  can  see  it  destroy- 
ing all  desire  of  conflict  and  victory ;  we  can  see  it 


A    VIEW  IN  PERSPECTIVE.  159 

doing  .away  with  race  prejudice,  and  we  can  see 
a  united  mankind  advancing  to  higher  and  higher 
planes.  Thus  it  is  that  the  intelligence,  conscience, 
and  social  habits  of  man,  and  above  all  his  approxi- 
mation toward  the  law  of  Christ,  have  produced  a 
new  era  in  the  history  of  life,  and,  as  a  result,  the 
development  of  mankind  in  the  future  is  not  to  run 
parallel  to  the  development  of  animals  in  the  past. 
No  longer  are  we  to  find  a  divergence  and  produc- 
tion of  numerous  species  of  animals  or  even  numer- 
ous species  of  intelligence.  There  is  to  be  one 
human  race,  a  race  of  marvellous  complexity,  ad- 
vancing to  higher  and  higher  planes.  Mankind  is 
to  remain  a  unit,  and  so  long  as  his  chief  character 
is  the  development  of  his  intellect,  he  will  still  con- 
tinue to  be  man,  whatever  be  the  changes  that  the 
future  may  see  either  in  his  physical  or  mental 
attributes. 

Each   Type  a  Master  of  the  Preceding. 

Before  leaving  this  sketch  on  the  outline  of  history, 
there  are  several  other  points  of  general  interest  to 
be  mentioned.  First  we  must  notice  that  in  all  of 
this  history  the  predominant  animal  type  of  any  age 
is,  as  a  rule,  more  than  master  for  all  of  the  animals 
that  have  preceded  it.  With  their  hard  shells  the 
mollusks  have  no  fear  of  the  lower  animals.  The 
activity  of  the  articulates  makes  them  more  than  a 
match  for  the  mollusks  or  other  lower  animals.  The 
vertebrates  are  always  superior  to  the  invertebrates, 
and  so,  as  a  rule,  the  amphibians,  reptiles,  birds,  and 
mammals  are  seen  in  turn  possessed  of  powers  that 


l6o  THE  LIVING  WORLD. 

make  them  masters  of  all  the  lower  forms.     Finally, 
man  supersedes  them  all. 

The  Rarity  of  Terrestrial  Life. 

Again  we  must  notice  how  few  are  the  types 
of  animals  that  have  ever  succeeded  in  adapting 
themselves  to  a  life  on  the  land.  Beyond  the 
vertebrates  and  the  air-breathing  articulates  (insects, 
spiders,  etc.),  there  are  almost  no  land  animals. 
Among  the  mollusks  there  are  a  few  snails  which 
have  become  adapted  to  a  thoroughly  terrestrial 
life.  There  are  indeed  quite  a  number  of  species  of 
snails  thus  breathing  air,  but  they  are  all  closely 
allied  to  each  other,  and  commonly  have  a  much 
restricted  habitat.  Evidently,  then,  only  a  few 
mollusks  have  really  been  able  to  acquire  the  power 
of  breathing  air.  None  of  the  other  invertebrates 
have  been  able  to  acquire  the  power  of  living  in  the 
air,  and  we  may  say,  therefore,  that  there  are  only 
two  types  of  animal  structure  which  are  adapted  to 
life  out  of  the  water.  The  Protozoa  all  require  water 
as  a  medium  through  which  they  can  feed  and  carry 
on  respiration.  Ccelentera  have  a  body  too  soft  to 
resist  gravity,  unless  assisted  by  the  buoyancy  of  the 
water.  The  same  may  be  said  of  the  Echinodermata, 
together  with  the  fact  that  their  characteristic  sys- 
tem of  organs,  the  water  system,  requires  the  presence 
of  water  to  make  it  of  any  value.  The  mollusks  are 
too  clumsy,  as  a  rule,  to  be  adapted  to  terrestrial 
life.  The  Vermes  mostly  respire  through  the  surface 
of  the  body  or  by  gills,  and  either  method  makes  it 
necessary  for  them  to  have  the  exterior  of  their  body 


A    VIE  IV  IN  PERSPECTIVE.  l6l 

constantly  moist.  They  are  therefore  aquatic,  or 
occasionally  live  in  moist  earth.  The  Articulataand 
Vertebrata  alone  are  well  adapted  to  a  terrestrial 
life,  and  of  these  immense  groups  only  five  classes 
have  really  become  terrestrial  animals  (Insecta, 
Arachnida,  Reptila,  Aves,  Mammalia).  Five  classes, 
then,  out  of  the  whole  animal  kingdom  are  all 
that  have  acquired  the  power  of  living  in  the  air. 
These  classes  have  more  to  contend  with  than  the 
aquatic  animals,  since  they  have  gravity  to  resist, 
and  must  accommodate  themselves  to  the  climate  ; 
they  must  adapt  themselves  to  varying  temperature, 
and  to  many  other  conditions  to  which  aquatic  ani- 
mals are  not  subjected.  Now  it  is  a  law  of  nature 
that  the  being  which  surmounts  the  greatest  obsta- 
cles is  the  one  to  rise  to  the  highest  plane.  It  is  no 
wonder,  then,  that  these  five  classes  of  animals  have 
become  predominant  types,  and  surpass  the  other 
animals  in  variety  and  numbers.  They  have  had 
the  whole  land  in  their  possession. 

We  cannot  definitely  say  when  the  terrestrial  fauna 
appeared.  In  the  Silurian  (2)  there  were  certainly 
in  existence  some  scorpions,  and  probably,  therefore, 
'other  land  animals.  Indeed,  traces  of  insects  have 
been  found  in  these  rocks.  Terrestrial  vertebrates 
did  not  appear  until  the  end  of  the  Carboniferous  (4), 
and  not  in  any  very  great  abundance  until  the  Meso- 
zoic  (5-8).  From  the  beginning  of  the  Mesozoic, 
however,  terrestrial  fauna  has  played  the  most  impor- 
tant part  in  the  history  of  the  world,  and  in  the  more 
recent  times  it  has  been  the  terrestrial  forms  of  life  to 
which  evolution  has  been  chiefly  confined. 


1 62  THE  LIVING  WORLD. 

It  is  also  a  fact  of  great  interest  that  both  of  these 
types  of  animals  which  acquired  terrestrial  habits,  the 
vertebrates  and  articulate  types,  have  ended  their 
history  with  the  development  of  mind.  The  devel- 
opment of  insects  has  ended  in  the  production  of  the 
complicated  insects  familiar  to  every  one  acquainted 
with  the  habits  of  the  bees  and  ants.  The  develop- 
ment of  the  vertebrates  produces  the  intelligence 
which  has  reached  its  culmination  in  man.  Both 
types  of  terrestrial  animals  have  developed  the  ner- 
vous system,  and  in  each  type  the  mental  nature  is 
the  special  character  of  the  highest  orders.  It  will 
also  be  noticed  that  the  development  of  instincts 
among  insects  has  been  entirely  independent  of  the 
development  of  the  intelligence  of  the  vertebrates. 
The  two  have  not  even  progressed  in  parallel  lines. 
Each  group  has  developed  the  functions  of  the  ner- 
vous system,  but  one  has  developed  the  reflex  func- 
tions to  an  extreme  (instinct),  and  the  other  the 
reasoning  powers.  The  intelligence  of  the  verte- 
brates cannot  then  be  regarded  as  a  more  highly 
developed  condition  of  the  instincts  of  insects, 
although  both  intelligence  and  instinct  are  func- 
tions of  the  mental  nature.  That  terrestrial  life  has 
had  the  effect  of  stimulating  both  types  of  land 
animals  into  the  development  of  two  distinct  types 
of  mind,  becomes  therefore  a  matter  of  even  greater 
interest. 

The  Modern  Fauna  an  Impoverished  One. 

Finally,  we  must  notice  that  the  present  age  is  one 
of  short  duration  and  comparatively  impoverished 


A    VIEW  IN  PERSPECTIVE.  163 

fauna.  The  Quaternary  (10)  has  certainly  lasted  a 
great  many  thousands  of  years,  but  in  comparison 
with  the  immense  periods  of  the  earlier  ages,  it  is 
very  short.  It  was  with  the  Quaternary,  indeed  with 
the  later  part  of  the  Quaternary,  that  the  strictly 
modern  fauna  (i.  e.  the  modern  species)  appeared, 
and  we  may  say,  therefore,  that  the  duration  of  the 
present  geological  age  is  very  short  compared  with 
the  immense  periods  of  time  that  have  preceded  it. 

The  modern  fauna  is  regarded  as  an  impoverished 
one.  Of  course  there  seem  to  be  more  animals  and 
more  species  to-day  than  ever  before,  but  this  is  due 
largely  to  the  fact  that  we  have  the  animals  them- 
selves to  study  to-day  in  profusion,  while  of  the  past 
we  have  only  here  and  there  a  specimen.  In  variety  of 
form  the  geological  ages  certainly  surpassed  the  pres- 
ent. There  are  but  few  classes  in  which  there  has  not 
been  such  a  great  extinction  of  orders  in  the  past 
that  the  representatives  remaining  are  to  be  regarded 
as  fragments.  Most  of  the  invertebrates  reached 
their  culmination  in  the  geological  ages,  and  many 
of  them  have  been  for  a  long  time  on  the  decline. 
Even  of  the  vertebrates,  every  class  has  been  in  much 
higher  state  of  development  in  the  past  than  at 
present.  The  fishes  belonged  to  the  Devonian  (3), 
though  one  order  has  subsequently  greatly  expanded 
since  then  (bony  fishes,  in  the  Cretaceous  (8).  The 
amphibians  belonged  to  the  Triassic  (6),  the  reptiles 
to  the  Jurassic  (7),  while  the  mammals  existed  in 
greater  profusion  in  the  Tertiary  than  they  do  to- 
day. More  than  one  third  of  the  orders  of  the  ver- 
tebrates have  become  extinct,  and  of  those  that 


164  THE  LIVING  WORLD. 

remain  a  larger  number  have  become  reduced  to  a 
few  unimportant  representatives  of  orders  which 
in  former  times  were  abundant  in  number  and  diver- 
sity of  form.  Some  of  the  orders  that  are  still  left,  it 
is  true,  are  perhaps  more  abundant,  so  far  as  number 
of  species  is  concerned,  than  any  orders  of  the  past, 
but  as  a  whole  the  fauna  of  to-day  consists  of  rem- 
nants of  the  past. 

Summary  of  the  Last  Two  Chapters. 

We  may  compare  the  history  of  the  whole  animal 
world  to  the  growth  of  a  giant  tree.  As  members  of 
the  human  race  our  position  is  among  the  topmost 
branches,  and  it  is  only  by  peeping  down  through  the 
foliage  that  we  dimly  get  an  idea  of  the  rest  of  the 
tree.  The  foliage  is  dense  and  our  vision  is  obscured. 
Absorbed  in  that  which  immediately  surrounds  us  we 
fail  to  see  the  size  and  magnificence  of  this  tree  of 
life,  and,  indeed,  not  infrequently  we  are  inclined  to 
believe  that  man  stands  alone,  having  nothing  to  do 
with  the  rest  of  the  tree.  Only  by  shutting  our  eyes 
to  the  dense  foliage  that  surrounds  us,  and  by  study- 
ing the  past  do  we  learn  that  mankind  too  is  a  mem- 
ber of  the  same  tree  of  life  that  has  lived  through  the 
ages  and  has  suffered  so  much  from  the  storms  of 
the  centuries  to  make  room  for  his  final  appearance. 

If  we  can  imagine  ourselves  as  removed  from  our 
natural  position  among  the  branches  and  viewing 
this  tree  of  life  in  perspective  from  the  distance,  it 
will  appear  something  as  follows.  Its  trunk  is  hid- 
den from  our  view  by  the  primeval  mists  of  the  early 
ages,  though  its  existence  in  the  dim  past  can  be  in- 


A    VIEW  IN  PERSPECTIVE.  165 

ferred  from  the  convergence  of  the  great  branches  as 
they  approach  this  region  of  obscurity.  All  above 
the  top  of  the  trunk,  however,  is  in  sight,  and  we 
can  see  the  tree  growing  with  an  ever  widening  ex- 
panse of  its  branches.  But  the  storms  of  the  ages 
have  played  great  havoc.  Many  limbs  have  been 
torn  off  completely,  many  more  have  been  so  shat- 
tered that  only  a  remnant  is  left  to  mark  the  position 
of  a  once  mighty  member.  The  prunings  that  have 
thus  occurred  have  frequently  served  to  give  more 
room  to  the  branches  that  are  left,  and  these  have 
taken  advantage  of  the  opportunity  to  develop  large 
numbers  of  small  twigs  and  fill  the  space  formerly 
occupied  by  a  fallen  member.  Occasionally  we  see 
a  branch  that  has  grown  with  marvellous  rapidity  for 
a  short  time  and  then  suddenly  died  ;  or  another 
that  grew  for  a  long  period  as  a  single  trunk  and 
then  suddenly  expanded  into  minor  branches  and 
twigs.  This  tree  of  life  is  old  and  most  of  it  has 
long  since  spent  its  energy.  Many  of  the  branches 
are  dead,  while  others  continue  to  live  only  in  the 
shape  of  scraggy  twigs.  Death  and  destruction  have 
played  such  havoc  that  the  tree  has  become  very  un- 
symmetrical  and  seems  from  our  distant  view  almost 
a  wreck.  But  still  some  of  the  branches  that  are  left 
alive  are  very  vigorous,  and  when  we  look  simply  at 
the  top  of  the  tree  the  vigor  displayed  there  almost 
conceals  from  us  the  wreck  of  the  past.  There  at 
the  top  we  see  a  single  branch  of  this  venerable  tree 
that  has  in  recent  times  begun  to  grow  to  an  enor- 
mous size.  A  new  law  regulating  its  growth  has 
stimulated  it  in  a  new  direction  and  caused  it  to  de- 


1 66  THE   LIVING  WORLD, 

velop  foliage  at  the  expense  of  branches.  So  rapidly 
is  this  branch  growing  to-day  that  it  bids  fair  to  ab- 
sorb into  itself  all  of  the  vitality  of  the  whole  trunk, 
crowding  out  of  existence  its  neighbors  and  allowing 
only  such  of  the  lower  branches  to  exist  as  do  not 
come  in  direct  conflict  with  it.  This  vigorous  branch 
is  man,  and  although  it  is  far  above  all  the  others, 
although  it  has  expanded  far  more  than  the  rest  and 
grown  so  far  away  from  the  trunk  that  its  connection 
with  the  great  tree  of  life  is  sometimes  obscured, 
still  our  study  by  the  light  of  history  shows  us  that 
this  branch,  too,  is  part  of  the  same  tree  to  which 
other  animals  belong,  and  is  simply  the  crowning 
top  of  this  tree  of  the  ages. 


CHAPTER  VII. 

HISTORY    OF   PLANTS. 

THE  history  of  the  plant  kingdom  from  early  ages 
to  the  present  time  requires  but  brief  notice.  The 
problems  connected  with  this  kingdom  are  much 
simpler  than  those  relating  to  animals.  While  plants 
certainly  constitute  important  factors  in  the  develop- 
ment of  life,  their  relation  to  the  history  of  mankind, 
which  is  after  all  the  primal  object  of  study,  is  very 
distant.  Like  animals  they  have  had  a  history,  and 
it  is  one  of  even  more  constant  progression.  Very 
early  in  the  history  of  life  plants  became  separated 
from  animals  by  acquiring  the  power  of  utilizing 
sunlight  as  a  source  of  energy,  and  though  some  of 
them  have  subsequently  lost  this  power,  it  is  never- 
theless probable  that  this  was  the  real  point  of 
separation  of  the  two  kingdoms.  Whether  plants 
or  animals  were  the  first  to  appear  cannot  be  de- 
termined, though  it  is  certain  that  animals,  as  they 
exist  to-day,  could  not  have  preceded  plants.  We 
have  seen,  however,  that  probably  neither  of  them 
preceded  the  other,  and  that  the  first  organic  life 
was  neither  animal  nor  plant. 

Of  the  early  history  of  plants  we  know  little  or 
nothing.  The  general  simplicity  of  their  structure 

167 


1 68  THE  LIVING  WORLD. 

and  their  lack  of  any  complicated  system  of  organs 
makes  their  embryological  history  less  significant 
than  that  of  animals.  We  can,  it  is  true,  determine 
that  all  plants  start  their  history  as  single  cells,  and 
this  would  seem  to  indicate  that,  like  animals,  they 
have  been  originally  derived  from  unicellular  ances- 
tors. But  it  is  impossible  to  find  traces  of  anything 
in  the  history  of  plants  that  correspond  to  the 
Gastraea  of  the  animal  world,  nothing  that  can  be 
regarded  as  the  central  starting-point  from  which  the 
various  groups  diverged.  Indeed,  the  history  of 
plants  seems  more  like  the  history  of  a  single  de- 
veloping line  of  life  than  of  a  series  of  diverging 
lines. 

Although  embryology  gives  us  little  help  in  study- 
ing the  history  of  plants,  still  by  combining  its 
teachings  with  the  facts  derived  from  comparative 
anatomy,  some  few  points  can  be  determined  in  re- 
gard to  the  line  of  descent  through  which  plants 
have  come.  That  the  unicellular  plants  were  fol- 
lowed by  the  low  algae  cannot  be  questioned.  That 
the  higher  plants  were  derived  from  these  seems 
also  sure.  The  mosses,  ferns,  club  mosses,  cycads, 
cone  bearers  (pines),  and  angiosperms  (common 
flowering  plants)  follow  each  other  in  something  like 
the  order  given,  with  increasing  grades  of  structure. 
Now  in  the  embryology  of  the  cone  bearers  (gym- 
nosperms)  we  can  still  find  traces  of  an  earlier  stage 
in  the  history  of  plants  corresponding  to  the  club 
mosses,  and  even  in  the  highest  plants  of  all,  the 
angiosperms,  there  are  indications  of  a  like  history. 
It  is  significant  to  find  that  the  fossil  history  of 


En 


Cr 


FIG.  21.     Diagram  illustrating  the  history  of  plants—/5  Pteridophyta.      Ly  Ly- 
copoda.      G  Gymnosperma.      Ex  Exognens.      En  Endogens.      A  Algsc. 

I69 


I/O  THE  LIVING  WORLD. 

plants  tells  somewhat  of  a  similar  story,  and  we  may 
thus  conclude  that  even  in  plants,  embryology  re- 
peats past  history,  at  least  in  a  measure.  Of  the 
early  history  of  plants,  however,  we  know  practically 
nothing,  and  of  the  history  of  later  ages  it  is  neces- 
sary to  combine  all  sources  of  evidence  in  order  to 
get  anything  like  a  connected  account. 

Plants  have  not  left  so  complete  a  fossil  record  as 
have  animals.  Particularly  is  this  true  of  the  lower 
forms  which  almost  universally  agree  in  having  no 
hard  parts  adapted  for  preservation.  So  imperfect 
is  their  preservation  that  it  is  impossible  in  many 
cases  to  determine  whether  a  given  specimen  in 
the  earliest  rocks  is  a  plant  or  simply  a  crystalli- 
zation, a  worm  track,  or  mud  crack.  There  is  no 
doubt  that  plants  were  in  existence  during  the  long 
Archean  (i)  age.  The  immense  beds  of  graphite 
belonging  to  these  times  give  us  almost  certain 
proof  of  the  fact.  But  no  traces  of  them  except 
the  graphite  beds  have  survived  the  metamorphosis 
of  the  rocks. 

In  the  Silurian  (2)  age,  however,  plants  were  un- 
doubtedly abundant,  and  our  first  record  of  them  is 
thus  nearly  contemporaneous  with  that  of  animals. 
But  the  plants  were  all  of  the  lower  types. 

Marine  algae  were  doubtless  in  great  numbers  in  the  seas  of  this 
period,  and  everything  seems  to  indicate  that  they  were  not  unlike 
their  descendants  of  to-day,  which  form  the  slimes  of  fresh  water  and 
some  of  the  sea  mosses.  Their  poor  preservation  makes  it  impossible 
to  say  much  about  them.  During  the  Silurian  (2)  there  were  also  in 
existence  at  least  two  types  of  land  plants,  living  perhaps  in  swamps 
or  shallow  water.  There  was  a  delicate  little  plant  named  Psilophy- 
ton,  thought  to  be  a  link  between  two  later  groups  (rhizocarps  and 


HISTORY  OF  PLANTS.  I/I 

lycopods),  and  also  a  plant  of  a  size  so  large  as  to  resemble  a  tree, 
though  in  structure  of  so  low  a  type  as  to  belong  to  the  algae  (Nema- 
tophyton).  Other  land  plants  were  living  also,  but  they  were  all  of 
a  very  low  type.  The  land  vegetation  was  marvellously  different 
from  that  of  to-day,  and  we  may  almost  look  upon  it  as  a  flora  of 
marine  algae  which  had  been  transferred  to  the  land  and  then  en- 
larged. Still  there  was,  even  at  this  time,  a  differentiation  into  stem 
and  leaf,  and  this  does  not  occur  in  ordinary  algae.  The  Silurian 
flora  was  in  one  marked  respect  very  different  from  the  Silurian 
fauna.  The  latter  has  surprised  us  with  its  diversity  and  high 
grade  of  development,  while  the  former  may  equally  surprise  us  with 
its  scarcity  and  its  low  grade.  The  development  of  plants  had  not 
reached  such  a  high  state  as  the  development  of  animals  at  this  time. 

In  the  next  age  (Devonian)  there  was  an  undoubted  advance. 
Algae  were  still  in  abundance,  as  indeed  they  have  been  in  all  ages 
up  to  the  present  time.  But  with  the  Devonian,  undoubtedly,  higher 
plants  appeared.  True  rhizocarps  and  lycopods  (horse  tails)  were 
abundant  at  this  time,  though  possibly  they  may  have  begun  in  the 
previous  age.  In  the  Devonian  also  we  find  true  ferns,  some  of  them 
small  and  others  of  large  size  like  our  tree  ferns.  In  this  age,  too, 
appeared  the  first  indication  of  the  flowering  plants  in  the  form  of 
gymnosperms  or  conifers  (yews  and  cordiates,  an  extinct  family). 
The  flora  of  the  Devonian  was  thus  on  the  whole  composed  of  the 
highest  of  the  cryptogams  (lycopods)  and  the  lowest  of  the  phanero- 
gams (evergreens  and  conifers). 

In  the  interval  between  the  Devonian  (3)  and  the  Carboniferous  (4) 
ages  there  were  great  changes  in  the  level  of  the  land.  After  various 
submergences  it  finally  arose  clothed  with  a  flora  of  precisely  the 
same  general  character  as  that  of  the  Devonian,  but  with  new  species 
and  genera.  The  changes  had  been  sufficient  to  obliterate  old  and 
produce  new  ones.  The  flora  still  consisted  of  the  same  groups  of 
plants  with  no  advance  in  structure.  The  ferns  became  very  diversi- 
fied, and  the  horse  tails  and  lycopods  reached  their  greatest  size,  but 
there  was  no  advance  in  structure.  The  age  was  characterized,  how- 
ever, by  the  great  development  of  the  cone  bearers  (Gymnosperma), 
ferns  (Pteridophyta),  and  the  horse-tail  group  (Lycopoda).  The  Car- 
boniferous was  an  age  of  especially  abundant  vegetation,  and  to  the 
plant  growth  of  that  time  we  owe  our  beds  of  coal. 

Coming  now  into  later  periods,  we  find  with  the  beginning  of  the 
Mesozoic  again  a  new  flora,  but  again  no  advance  in  structure  of  much 


172  THE  LIVING  WORLD. 

importance.  The  typical  vegetation  of  the  Carboniferous  became  less 
prominent  and  the  cycads  appeared.  The  cycads  are  the  prominent 
type  of  the  Mesozoic.  During  the  passage  of  the  Mesozoic,  however, 
we  see  a  change  in  the  flora  of  a  radical  sort.  Even  with  the  Trias- 
sic  (6)  the  giant  horse  tails  began  to  wane,  and  the  more  modern  forms 
of  moderate  size  appeared.  In  the  Jurassic  (7)  the  cone-bearing  plants 
reached  their  highest  development,  from  which  culminating  point 
they  have  steadily  declined.  Some  of  our  modern  forms  arose  at  this 
time,  the  genus  Pinus  being  in  considerable  abundance.  During  the 
Jurassic  also  the  first  of  the  endogens  made  their  appearance  in  the 
form  of  screw  pines  and  grasses,  and  a  little  later  the  palms  appeared. 

The  Cretaceous  (8)  period  is,  however,  the  line  that  marks  the 
boundary  between  the  older  vegetable  world  and  the  modern,  just  as 
the  beginning  of  the  Mesozoic  separated  the  older  animals  from  the 
modern  forms.  Occasional  traces  of  the  higher  flowering  plants  are 
found  in  the  lower  rocks  of  the  Cretaceous,  but  it  is  in  those  of  the 
upper  Cretaceous  that  they  appear  in  abundance.  Here  there  sud- 
denly bursts  upon  our  view  an  abundant  flora  of  modern  forms. 
Most  of  the  plants  of  that  time  belonged  to  the  same  genera  as  those 
living  to-day,  and  many  of  them  were,  so  far  as  we  can  tell,  of  the 
same  species.  Our  knowledge  of  them  is  chiefly  confined  to  the 
preserved  leaves,  and  these  are  a  rather  unsatisfactory  basis  for  deter- 
mination of  species,  but  there  seems  to  be  no  doubt  that  the  poplars, 
willows,  beeches,  oaks,  birches,  alders,  laurels,  sassafras,  magnolias, 
butternuts,  hickorys,  and  many  others  were  represented  in  profusion. 
There  is  also  pretty  good  evidence  of  the  existence  of  the  higher 
flowering  plants,  like  the  Composite,  at  this  time,  though  doubtless 
the  plants  with  inconspicuous  flowers  predominated.  In  short,  at 
this  time  the  modern  botanical  world  was  nearly  complete,  so  far  as 
type  was  concerned.  Its  highest  forms  had  been  developed  even 
then,  and  from  that  time  to  the  present  the  growth  has  been  simply 
in  the  expansion  of  typesUhen  existing,  and  in  the  relative  increase 
of  the  numbers  of  the  highest  flowering  plants.  The  gymnosperms 
certainly  have  become  less  abundant,  and  the  plants  with  large  con- 
spicuous flowers  have  greatly  increased  in  preponderance. 

•The  endogens  have  remained  nearly  constant  since  then,  so  that 
we  may  say  that  in  the  later  Cretaceous  the  vegetable  world  reached 
nearly  as  high  a  position  as  it  has  to-day. 

We  may  now  ask  whether  in  this  history  of  plants  we  can  trace 
the  origin  of  the  different  groups  of  plants  from  each  other.  As 


HISTORY  OF  PLANTS.  1/3 

already  pointed  out,  the  similarity  in  structure  would  lead  us  to  be- 
lieve that  the  Silurian  land  plants  were  simply  terrestrial  forms  of 
the  marine  algae.  The  same  conclusion  would  follow  from  the  study 
of  their  anatomy  and  development.  The  rhizocarps,  to  which  some 
of  the  early  land  plants  seem  to  be  related,  are  themselves  closely 
related  to  various  algae.  So,  too,  may  the  ferns  and  horse  tails 
or  the  Carboniferous  be  regarded  as  derived  from  lower  marine 
plants  (algce),  with  various  modifications,  probably  through  some 
lost  intermediate  steps.  The  cone  bearers,  however,  give  evidence 
of  having  descended  not  from  the  algse,  but  from  some  high  form 
of  plant  belonging  to  the  Lycopoda.  Their  structure,  and  espe- 
cially their  method  of  reproduction  and  development,  shows  them 
closely  related  to  certain  plants  classed  with  the  Lycopoda  (Isoetes), 
but  differing  from  the  ordinary  Lycopoda  in  having  two  kinds  of 
spores,  a  large  one  and  a  small  one.  These  micro-  and  macrospores 
correspond  to  the  pollen  and  embryo  sac  of  the  flowering  plants,  and 
from  some  such  source  doubtless  have  the  latter  been  produced. 
The  endogens  again  were  unquestionably  not  products  of  the  cone 
bearers,  but  they  had  an  independent  origin  down  the  main  line,  not 
unlikely  an  origin  close  to  that  of  the  gymnosperms.  The  higher 
flowering  plants,  the  exogens,  though  closely  related  to  the  gym- 
nosperms, were  probably  not  derived  from  them  directly,  and  were 
certainly  not  derived  from  the  endogens.  Probably  they  had  an 
independent  origin  from  some  lower  group  close  to  the  gymnosperms. 
It  would  seem  then  that  some  low  algae-like  type  of  plant  at  one  time 
became  terrestrial,  and  then  there  occurred  a  divergence  from  it 
which  resulted  in  the  various  forms  of  cryptogams,  the  mosses,  ferns, 
and  lycopods.  These  groups  soon  expanded  and  reached  their  cul- 
mination in  the  Carboniferous.  But  along  one  line  of  descent 
(Isoetes,  etc.,)  two  kinds  of  spores  were  produced,  and  this  line  now 
expanded  and  produced  the  flowering  plants,  the  gymnosperms, 
endogens,  exogens,  all  of  which  plants  retain  the  t\vo  kinds  of 
spores  (pollen  and  embryo  sac),  but  seem  to  give  indication  of  having 
had  independent  origins  from  the  cryptogams. 

It  is  a  fact  of  no  little  interest  that  the  flora  of  the  world  probably 
did  not,  as  would  be  expected,  originate  in  the  tropical  regions,  but 
on  the  contrary  in  the  Arctic  zone.  Abundant  evidence  shows  that 
in  the  northern  regions  the  various  groups  of  plants  first  appeared 
and  culminated.  This  must,  of  course,  indicate  that  the  climate  of 
the  Arctic  zone  was  not  the  frigid  one  that  it  is  to-day. 


THE  LIVING  WORLD. 

The  first  point  that  strikes  out  attention  in  this 
history,  is  that  the  life  of  plants  has  been  more  evi- 
dently one  of  progression  than  that  of  animals.  We 
do  not  find  that  the  Silurian  (2)  age  opened  with  an 
abundant  and  highly  specialized  flora  to  correspond 
to  the  highly  developed  fauna  of  the  times.  The 
plants  were  few  and  all  of  the  lower  orders.  Com- 
paratively speaking,  then,  we  may  say  that  the  ani- 
mal kingdom  had  reached  a  much  higher  state  of 
development  by  the  beginning  of  the  Silurian  than 
the  vegetable  kingdom.  While  all  of  the  classes  of 
animals  (except  two)  had  appeared  at  this  time  or 
during  the  Silurian  age,  there  were  no  plants  higher 
than  rhizocarps,  and  nearly  all  of  the  plants  in  exis- 
tence were  algae  or  still  lower  types.  The  whole 
vegetable  world  which  is  familiar  to  us  to-day  was 
still  to  be  developed. 

We  notice  next  that  although  in  the  early  periods 
the  animal  world  developed  faster  than  the  vegetable 
world,  still  we  find  that  before  the  close  of  the  geo- 
logical ages  the  relation  was  reversed.  The  vegeta- 
ble kingdom  reached  its  culmination  long  before  the 
animal  kingdom.  With  the  Cretaceous  (8),  the  vege- 
table world  had  developed  its  highest  types,  the  sub- 
sequent history  being  only  an  elaboration  of  them. 
But  at  that  time  the  highest  class  of  animals  had  not 
appeared  at  all,  for  the  true  mammals  came  into 
existence  only  in  the  next  age  (Tertiary),  and  very 
great  advance  took  place  in  them  even  later. 

In  general  it  remains  to  be  noticed  that  with  the 
opening  of  the  Silurian,  the  vegetable  kingdom  had 
reached  a  condition  where  cellular  differentiation  was 


HISTORY  OF  PLANTS.  1/5 

found.  All  the  powers  pertaining  to  the  cell  seemed 
to  be  fixed,  for  there  is  no  reason  for  thinking  that 
the  cells  of  early  plants  were  not  about  like  those  of 
to-day.  During  the  Silurian  also  the  stem  and  leaf 
were  differentiated.  Sexual  reproduction  probably 
had  begun  at  this  time,  but  it  was  in  a  low  form,  and 
the  formation  of  fruit  did  not  appear  until  later. 
Thus  the  advance  has  been  almost  wholly  in  refine- 
ments of  parts,  and  not  in  the  productions  of  new 
features. 

Finally,  we  notice  that  it  is  impossible  at  present 
to  trace  the  various  types  in  the  vegetable  kingdom 
to  a  common  centre.  We  do  not  find  any  evidence 
of  a  rapid  divergence  from  one  central  point  of  origin. 
In  the  animals  we  have  reason  for  thinking  that  all 
of  the  great  types  arose  early  as  branchings  from 
one  simple  type,  the  Gastraea,  and  that  the  different 
sub-kingdoms  did  not  arise  from  each  other  to  any 
noticeable  extent.  In  the  vegetable  kingdom  the 
reverse  seems  to  be  nearer  the  truth.  New  types  of 
plants  were  constantly  arising  till  the  Cretaceous, 
and  there  is  every  reason  for  thinking  that  they 
arose  from  the  earliest  existing  plants  in  all  cases. 
In  other  words,  the  history  of  the  vegetable  world 
is  rather  to  be  compared  to  the  history  of  a  single 
sub-kingdom  of  animals  than  to  the  history  of  the 
whole  animal  kingdom.  Like  the  vertebrates  they 
have  had  most  of  their  development  since  the  Silu- 
rian age,  although  it  is  true  that  their  history  really 
began  earlier.  In  the  fossil  history  of  plants,  there- 
fore, we  find  just  such  an  advance  as  we  have  seen 
in  the  vertebrates,  and  not  such  a  mixture  of  ad- 


THE  LIVING  WORLD. 

vance  and  stationary  condition  as  we  have  seen  in 
the  animal  world  taken  as  a  whole.  This  difference 
is  certainly  a  striking  one,  as  we  take  a  cursory 
glance  at  the  fossil  study  of  plants  and  animals  as 
they  are  known  to-day.  The  fact  may,  however,  be 
partly  due  to  our  incomplete  knowledge  of  the  lower 
orders  of  plants.  The  lowest  classes  of  plants  are 
almost  wholly  unrepresented  by  fossils,  and  through 
all  the  geological  ages  it  is  the  higher  classes  which 
have  been  most  preserved  and  most  studied.  Per- 
haps if  our  fossil  record  of  lower  plants  were  as  com- 
plete as  that  of  the  lower  animals,  we  should  find 
that  here  too  there  has  been  divergence  from  com- 
mon centres  to  a  much  greater  extent  than  now 
appears.  But  however  that  may  be,  the  facts  as 
collected  at  the  present  time  point  to  a  history  of 
much  more  continuous  progression  among  plants 
than  animals.  See  Fig.  21. 


CHAPTER   VIII. 

THE   FUTURE   OF   THE    LIVING   WORLD.* 

SCIENCE  is  at  all  times  trying  to  read  the  future 
by  means  of  the  past.  No  one  questions  the  right 
of  astronomy  to  make  predictions,  and  her  success 
in  this  direction  is  everywhere  recognized.  It  is, 
then,  certainly  a  legitimate  question  for  us  to  ask 
here  whether  the  past  history  of  life  cannot  give  us 
indications  of  the  direction  of  the  drift  of  the  living 
world,  and  thus  enable  us  with  something  like  prob- 
ability to  look  into  its  future.  Such  predictions 
cannot  of  course  claim  anything  like  the  certainty 
of  the  predictions  of  astronomy,  for  the  complexity 
of  the  problem  is  too  great.  At  the  same  time,  a 
little  study  will  show  that  there  are  some  definite 
results  plainly  indicated  by  the  drift  of  the  past,  and 
a  clearer  idea  of  the  meaning  of  past  history  can  be 
obtained  by  trying  to  see  in  what  direction  it  turns 
our  thoughts  for  the  future. 

We  have  just  seen  that  a  fair  idea  of  the  life  of  the 
world  can  be  obtained  by  comparing  it  to  a  giant 

*  The  ideas  advanced  in  this  chapter  were  first  published  in  the 
American  Naturalist,  1886,  from  which  part  of  the  following  pages 
are  quoted. 

ii  ,        177 


1/8  THE  LIVING  WORLD. 

tree.  In  this  comparison  we  have  seen  that  the  tree 
is  to  be  regarded  as  an  old  one,  all  of  whose  branches 
show  by  their  shattered  condition,  the  effects  of  the 
storms  of  the  ages.  Comparatively  few  branches 
remain  alive,  while  a  larger  number  have  either  dis- 
appeared or  become  reduced  to  a  few  still  vigorous 
shoots.  The  highest  branch  alone  appears  to  be  in 
its  primal  vigor,  still  rapidly  growing  and  expanding, 
and  this  because  of  the  influence  of  a  new  life  prin- 
ciple, perhaps  engrafted  into  the  old  tree. 

Now  if  such  a  comparison  is  a  correct  one,  it 
is  evident  that  the  tree  must  be  looked  upon  as 
being  near  its  death.  Of  course,  however,  it  is  pos- 
sible to  question  the  correctness  of  this  comparison, 
and  we  must  therefore  ask  whether  there  are  really 
any  grounds  for  believing  that  the  life  of  the  world 
has  passed  its  prime,  that  while  man  is  the  crowding 
creation,  his  appearance  indicates  the  decline  of  the 
living  world  as  a  whole. 

Development  More  Rapid  in  Early  Ages. 

In  order  to  answer  this  question  in  the  affirmative, 
it  will  only  be  necessary  to  refer  to  some  of  the  facts 
already  noticed  regarding  the  previous  ages.  First 
we  may  notice  again  the  significant  fact  that  the 
development  of  the  animal  kingdom  seems  to  have 
been  more  rapid  in  the  earliest  times  than  it  has 
been  in  subsequent  ages.  The  diversity  of  the  Silu- 
rian (2)  fauna  has  already  been  noticed,  and  this,  of 
course,  means  that  a  large  part  of  the  evolution  of 
type  had  occurred  previous  to  the  Silurian  age.  All 


THE  FUTURE   OF   THE  LIVING  WORLD.         Ijg 

subsequent  development  has  been  only  elaboration 
and  not  the  production  of  new  types. 

Now  we  are  not  at  liberty  to  assume  an  indefinite 
amount  of  time  prior  to  the  Silurian.  Of  course  it  is 
impossible  to  say  just  how  long  a  time  elapsed 
between  the  origin  of  life  and  the  beginning  of  the 
Silurian,  but  it  seems  hardly  possible  that  it  could 
have  equalled  the  time  that  has  elapsed  since  then. 
But,  upon  evolutionary  theories,  the  animal  kingdom 
must  have  developed  during  that  period  from  the 
lowest  unicellular  condition  to  the  complex  and 
diversified  fauna  of  the  Silurian.  When  we  consider, 
therefore,  that  during  this  time  all  of  the  important 
groups  of  the  animal  kingdom  arose  (with  perhaps 
the  exception  of  the  vertebrates)  and  that  none  have 
arisen  since  that  time,  it  becomes  quite  evident  that 
evolution  must  have  progressed  with  greater  rapidity 
at  that  time  than  it  has  since.  This  conclusion  is 
no  new  one,  for  many  naturalists  have  seen  the 
necessity  of  making  some  such  assumption.  It  will, 
indeed,  be  generally  acknowledged  that  evolution  in 
the  earliest  ages  was  more  rapid  than  at  present. 

Here,  then,  we  see  another  point  of  likeness  in  the 
comparison  of  the  living  world  with  the  life  of  the 
individual.  An  individual  when  it  begins  life  grows 
most  rapidly,  but  from  the  very  moment  of  its  birth 
the  rapidity  of  growth  lessens  until  a  stationary 
condition  is  reached  at  maturity.  So  it  seems  in 
the  longer  history  of  world  life.  Its  growth  was 
most  rapid  at  the  birth  of  life  and  has  been  decreas- 
ing since  that  time,  not  with  regularity,  perhaps, 
being  frequently  interrupted  by  periods  of  more 


180  THE  LIVING  WORLD. 

rapid  expansion,  yet  nevertheless  on  the  whole 
decreasing.  If  this  is  so,  it  plainly  implies  an  end 
to  the  process. 

The  Organic  World  Approaching  a  Limit. 

That  the  organic  world  is  approaching  a  limit  in 
its  development  is  a  conclusion  which  does  not,  how- 
ever, depend  upon  any  vague  idea  of  growth  for  its 
support ;  for  a  little  thought  upon  discovered  laws 
will  clearly  show  us  that  there  must  be  a  limit  to 
advance.  The  best  definition  which  has  ever  been 
given  to  the  grade  of  structure  of  animals  and  plants 
is  the  degree  to  which  differentiation  of  organs  is 
carried.  Evolution  as  it  tends  to  raise  the  grade  of 
animals  is  constantly  increasing  the  amount  of  differ- 
entiation and  specialization.  We  have  already  seen 
that  such  differentiation  and  specialization  becomes 
self-limited.  A  low  undifferentiated  and  unspecial- 
ized  organism  has  an  indefinite  possibility  in  its 
lines  of  specialization.  A  simple  spherical  cup  of 
cells,  the  supposed  common  ancestor  of  the  animal 
kingdom,  may  be  modified  in  a  very  great  variety  of 
directions,  each  of  which  may  give  rise  to  a  different 
type  of  animal.  This  possibility  lies  in  the  fact  that 
it  is  as  yet  undifferentiated  and  unspecialized.  But 
just  as  soon  as  it  does  become  modified  in  any  one 
direction  the  possibilities  decrease.  Some  of  the 
descendants  of  this  ancestor  becoming  vertebrates 
are  forever  precluded  from  becoming  anything  else ; 
others  becoming  mollusks  must  remain  mollusks 
forever,  with  all  of  their  descendants.  And  as  later 
descendants  become  further  modified  in  any  direc- 


THE  FUTURE   OF    THE  LIVING  WORLD.         l8l 

tion  into  definite  types  the  chance  for  future  modifi- 
cation becomes  rapidly  less.  It  is  only  the  abso- 
lutely undifferentiated  which  has  infinite  possibilities, 
for  as  soon  as  a  single  step  is  taken  in  any  direction, 
the  possibilities  become  finite.  Now  it  is  plain  that 
this  continued  specialization  cannot  go  on  forever. 
Since  evolution  does  not  retrace  its  steps,  every  step 
in  advance  limits  the  possible  lines  of  development. 
All  the  descendants  of  the  vertebrate  line  must 
conform  jto  the  vertebrate  type.  The  vertebrates 
become  separated  into  fish,  reptile,  and  mammal,  and 
each  group  is  still  further  fettered  in  its  develop- 
ment by  the  special  line  which  its  ancestors  have 
taken.  The  descendants  of  the  animals  which  have 
started  the  order  of  birds  cannot  take  any  new  line. 
They  can  develop  this  type  to  perfection,  or  they 
may  lose  their  special  characters,  but  there  they  must 
stop.  And  thus,  with  every  step  in  advance,  the 
possibilities  of  expansion  are  constantly  decreasing. 

Now  a  continued  specialization  of  this  sort  is  sure 
to  reach  a  limit  ;  it  must  run  to  extremes  and  event- 
ually stop.  Physical  laws  will  of  themselves  set 
limits  to  every  line  of  advance,  even  if  there  be  no 
such  limits  determined  by  the  organism  itself.  It  is 
easy  to  find  examples  which  will  show  that  such  has 
been  the  general  history  of  groups  in  the  past.  Some 
have  reached  the  extreme  of  their  development  in 
the  distant  past,  and  have  ceased  to  advance,  or, 
perhaps,  have  disappeared.  Others  seem  even  now 
to  be  at  the  summit  of  their  advance,  and  others 
still  are  yet  advancing.  The  line  of  development 
represented  by  the  trilobites  has  completely  ex- 


1 82  THE   LIVING  WORLD. 

hausted  itself.  It  rapidly  approached  its  limits  even 
in  the  Silurian  (2),  and  then  began  to  dwindle  away, 
and  has  disappeared  entirely.  The  brachiopods  had 
also  at  this  time  reached  their  point  of  highest 
specialization,  and  had  become  a  highly  developed 
group  even  at  this  early  age.  Since  then  they  have 
remained  stationary  as  to  their  organization,  having 
steadily  decreased  in  numbers,  and  the  few  that  are 
left  show  no  advance  over  the  Silurian  forms.  The 
cephalopod  mollusks  gradually  increased  in  com- 
plexity during  the  Paleozoic  (2-4)  ;  and  finally  a 
limit  of  the  shelled  forms  was  reached  in  the  ammon- 
ites of  the  Jurassic  (7)  and  Cretaceous  (8).  The 
culmination  was  followed  by  extinction.  Meantime 
a  second  line  of  development  began,  that  of  the 
naked  cephalopods  (squids,  cuttle  fishes),  and  this 
has  gone  on  advancing  until  the  present  time.  The 
decapod  Crustacea  represent  a  group  which  is  even 
now  near  its  culmination.  From  their  first  appear- 
ance in  the  Carboniferous  (4)  there  has  been  a  tend- 
ency to  a  concentration  of  organs  toward  the  head. 
As  this  specialization  advanced,  the  abdomen  be- 
came smaller,  while  the  head  region  became  larger. 
Finally,  in  the  crabs,  everything  was  concentrated 
in  the  head  region.  The  abdomen  remained  as  little 
more  than  rudiment.  Evidently  we  are  here  near  a 
limit,  and  we  may  look  upon  the  crabs  of  to-day  as 
the  culmination  of  the  special  line  of  development 
which  has  characterized  this  line  of  animals.  The 
vertebrates  in  general  have  been  continually  advanc- 
ing during  geological  times,  with  a  continued  increase 
in  specialization  and  in  multitude  of  types.  But  even 


THE  FUTURE   OF   THE   LIVING  WORLD.         183 

here  there  has  been  the  same  story  of  limitation. 
The  ganoids  culminated  in  the  Devonian  (3),  and 
have  advanced  no  farther.  One  great  line  of  reptiles 
reached  its  limit  in  the  Jurassic  (7).  And  so  every- 
where. The  study  of  every  group  teaches  that  the 
past  history  has  been  a  gradual  specialization  which 
approaches  a  limit.  In  many  cases  in  the  past  this 
limit  has  been  reached,  and  advance  has  ceased  ;  in 
others,  the  animals  are  still  on  the  road  toward  it. 

It  is  plain,  then,  that  unless  there  is  some  way  for 
new  lines  of  specialization  to  arise,  the  evolution  of 
the  animal  world  is  inevitably  approaching  its  end. 
With  every  advance  in  differentiation,  the  possible 
lines  of  development  decrease,  and  since  the  actual 
lines  followed  are  tending  to  run  themselves  out,  the 
whole  must  eventually  stop.  Is  it,  however,  possible 
that  new  lines  of  differentiation  may  arise,  and  thus 
the  development  of  the  living  world  go  on  in- 
definitely? 

New  Lines  of  Specialization  Not  Now  Appearing. 

First,  we  must  notice  that  the  development  of  the 
vegetable  kingdom  practically  ceased  long  ago.  As 
already  noticed,  the  Cretaceous  (8)  rocks  show  us 
representatives  of  the  highest  orders  of  plants.  At 
that  time  the  plants  in  existence  do  not  seem  to  have 
differed  materially  from  those  of  to-day,  since  many 
species  are  identical  so  far  as  can  be  determined. 
The  vegetable  kingdom  thus  practically  reached  its 
culmination  at  this  time  ;  for  although  many  new 
species  have  appeared  since  then,  there  has  been  no 
advance  in  type.  The  time  since  then  has  been  long 


1 84  THE  LIVING  WORLD. 

enough  certainly  for  a  great  amount  of  change,  if 
the  limit  had  not  been  practically  reached.  It  has 
been  sufficient  for  the  entire  development  of  the 
class  of  mammals,  with  its  great  profusion  ;  but  the 
plant  world  has  remained  nearly  stationary,  and  this 
suggests  to  us  that  we  may  look  for  no  farther  de- 
velopment along  this  line. 

With  the  animal  kingdom,  however,  it  is  different. 
Even  until  the  present  era  we  can  see  that  develop- 
ment of  new  and  higher  forms  has  continued.  Is 
there  any  indication  that  it  has  reached  its  end  ? 

That  there  is  a  theoretical  possibility  of  the  origin 
of  new  types  cannot  be  denied.  New  types,  i.  e.,  new 
lines  for  specialization,  can  arise  only  from  undiffer- 
entiated  forms.  But  such  undifferentiated  forms 
still  exist  in  great  numbers.  Even  the  most  un- 
specialized  forms  of  all,  the  unicellular  animals,  are 
abundant  enough,  and  in  all  groups  we  are  acquainted 
with  more  or  less  generalized  types.  Theoretically, 
then,  there  is  no  reason  wrhy  any  one  of  these  forms 
should  not  expand  itself,  and  thus  form  an  eternal 
source  of  new  world  forms.  So  long  as  the  un- 
specialized  forms  do  not  become  extinct,  we  cannot 
deny  the  possibility  of  an  infinite  number  of  future 
sub-kingdoms,  which  would  of  course  make  the 
animal  kingdom  an  example  of  never-ending  evolu- 
tion. But  all  of  our  evidence  indicates  that  such 
a  future  is  probably  not  a  practical  possibility, 
even  though,  so  far  as  we  can  see,  it  may  be 
a  theoretical  one.  All  biological  studies  point 
strongly  to  the  conclusion  that,  instead  of  several 
points  of  origin  the  animal  kingdom  has  had  only 


THE  FUTURE   OF    THE  LIVING  WORLD.         185 

one.  The  sub-kingdoms  have  not  arisen  independ- 
ently from  the  Protozoa,  but  have  all  had  a  common 
ancestor,  the  Gastnea,  and  this  means  that  only  once 
has  the  unicellular  form  given  rise  to  an  important 
line  of  multicellular  descendants  which  perpetuated 
itself.  Though  the  Coelentera  stand  very  near  this 
primitive  Gastraea,  there  is  no  evidence  that  the 
sub-kingdom  has  the  power  of  further  production  of 
new  types  ;  but,  on  the  contrary,  everything  tends 
to  show  that,  whatsoever  differentiation  of  this 
simple  type  ever  did  take  place  to  give  rise  to  the 
sub-kingdoms,  occurred  before  the  Silurian.  Since 
paleontology  shows  that  no  great  types  have  arisen 
since  the  Silurian,  it  is  plain  that  all  of  the  expansion 
of  the  simple  unicellular  form  must  have  taken  place 
before  the  Silurian.  And  coming  through  the  later 
ages,  we  find  that  the  evidence  is  the  same  in  its 
tenor.  The  conclusion  everywhere  seems  to  be  that 
when  a  generalized  form  has  given  rise  to  one  or  two 
lines  of  development,  it  either  disappears  or  loses  its 
power  to  originate  new  forms.  Every  bit  of  evi- 
dence which  indicates  a  fundamental  unity  of  the 
animal  kingdom  testifies  to  the  same.  Without 
questioning  the  theoretical  possibility  that  any  or 
all  of  the  existing  unspecialized  forms  may  in  the 
future  develop,  we  must  acknowledge  that  the 
probability  is  against  it.  Nothing  in  history  in- 
dicates that  these  groups  retain  the  power  to  ex- 
pand, and  there  is,  therefore,  no  reason  for  thinking 
it  a  possibility  in  the  future.  Remembering  what  a 
large  number  of  groups  we  are  learning  to  trace 
back  to  the  Silurian,  remembering  that  development 


1 86  THE  LIVING  WORLD. 

in  the  later  geological  ages  has  consisted  simply 
in  the  expansion  of  groups  appearing  long  before, 
we  must  conclude  that  the  power  of  the  undifferen- 
tiated  forms  to  expand  into  different  lines  of  devel- 
opment disappears  very  early  in  their  history. 
While  then  we  cannot  deny  the  possibility  of  an 
indefinite  future  development  from  the  existing 
generalized  types,  it  is  certainly  improbable  that 
any  new  great  groups  will  arise.  Man,  seizing  upon 
the  last  undifferentiated  faculty,  the  intellect,  is 
developing  this  to  the  extreme.  With  him,  the 
animal  world  proper  has  ended  and  the  intellectual 
world  begun. 

If  there  is  anything  further  needed  to  convince  us 
that  the  evolution  of  animals  ceases  with  man,  we 
have  only  to  notice  his  influence  upon  the  rest 
of  the  animal  kingdom.  We  cannot  yet  compute 
that  influence,  but  it  will  doubtless  be  the  death- 
blow to  the  evolution  of  animals.  Man  is  rapidly 
causing  the  extinction  of  almost  all  land  animals,  at 
least  the  larger  ones.  As  the  frontiers  of  civilization 
are  being  extended  farther  and  farther  into  the 
uninhabited  regions,  he  is  driving  out  of  existence 
all  of  the  large  animals  and  many  of  the  smaller 
ones.  We  have  only  to  look  ahead  a  comparatively 
short  time  to  see  the  extinction  of  nearly  all  land 
animals,  except  such  as  may  strike  man's  fancy  to 
use  or  preserve.  To  what  extent  this  may  apply  to 
other  animals — to  insects,  marine  animals,  etc. — is 
not  clear.  But  in  the  highest  group  of  animals,  the 
vertebrates,  it  is  pretty  clear  that  man  is  eventually 
to  bring  about  not  only  the  end  of  advance,  but  also 


THE  FUTURE   OF  THE   LIVING  WORLD.         l8/ 

the  practical  extermination  of  all  animals  except  such 
as  he  especially  preserves. 

His  mastery  over  the  higher  vertebrates  is  so  un- 
bounded that  he  is  the  only  one  who  has  any  possi- 
bility of  farther  advance.  With  the  lower  animals, 
his  competition  is  not  so  severe,  but,  as  we  have 
seen,  these  lower  types  have  practically  ended  their 
evolution  in  the  distant  past. 

To  man  alone,  then,  are  open  further  possibilities 
of  higher  development,  and  his  development  must 
be  wholly  mental.  His  unique  position  in  nature 
comes  upon  us  now  with  double  significance.  Even 
more  forcibly  than  ever  do  we  see  the  significance  of 
the  new  law  of  love  to  which  he  is  trying  to  adapt 
his  life.  Remembering  how  lines  of  specialization 
exhaust  themselves,  and  how  divergence  of  character 
ends  in  extremes,  we  begin  to  get  a  grand  conception 
of  the  law  of  love  which  binds  mankind  into  unity, 
thus  checking  the  development  of  castes  and  forcing 
him  to  advance  as  a  whole.  The  law  of  love  is  the 
only  law  which  could  produce  the  highest  mental 
and  moral  development. 


REFERENCES. 


A  long  list  of  references  would  be  inappropriate  in  this  work. 
The  following  short  list  may  be  of  value  to  those  wishing  further 
reading  upon  the  topics  treated  in  the  foregoing  pages  : 

PROTOPLASM    AND    CELLS. 

BERTHOLD.     "  Studien  ttber  Protoplasma  mechanik,"  Leipzig,  1887. 

NELSON,  JULIUS.     "Heredity  and  Sex,"  American  Naturalist,  1887. 

PASTEUR,  L.     "  Studies  on  Fermentation,"  Macmillan  &  Co.,  1879. 

SCHWARZ.  "  Morphological  and  Chemical  Composition  of  Proto- 
plasm," Zeitr.  z.  BioL  d.  Pflanzen,  v.  1887. 

WEISSMAN.  "  Essays  upon  Heredity  and  Kindred  Biological 
Problems,"  Clarendon  Press,  1889. 

WHITMAN,  C.  O.  "  Seat  of  Formative  and  Regenerative  Energy," 
American  Jour,  of  Morphology,  1 888. 

LIFE. 

BASTIAN,  CH.  "  The  Beginnings  of  Life,"  D.  Appleton  &  Co.,  1872. 
"  The  Modes  of  Origin  of  Lowest  Organisms,"  Macmillan 
&  Co.,  1871. 

BEALE,  SIR  LIONEL.  "The  Mystery  of  Life,"  J.  &  A.  Churchill, 
1871.  "Protoplasm;  or,  Matter  and  Life,"  Lindsay  &  Blakis- 
ton,  1874. 

DRYSDALE.   "  Protoplasmic  Theory  of  Life,"  Balliere,  Tyndall  &  Co. 

HUXLEY,  TH.  "  The  Physical  Basis  of  Life,"  Humboldt  Library; 
originally  delivered  as  a  lecture  in  1868.  "  Anatomy  of  Inver- 
tebrates," D.  Appleton  &  Co.,  1878.  Article  on  Biology 
in  Encyclo.  Britannica.  "Animal  Automatism,"  Fortnightly 
Review,  1874. 

189 


1 90  REFERENCE  S. 

MIXOT,  C.  S.      "On  the  Conditions  to  be  Filled  by  a  Theory  of 

Life,"  Proc.  of  Am.  Association,  1879. 

MORRIS,  C.  H.      "Organic  Physics,"  American  Naturalist,  1882. 
DuBois  REYMOND.     "  Seven  World  Problems,"  Pop.  Set.  Monthly, 

1882. 
TYNDALL,  J.     "  Floating  Matter  in  the  Air,"  D.  Appleton  &  Co., 

1882. 
WARD,  LESTER.     "  Organic  Compounds  in  their  Relations  to  Life," 

American  Naturalist,  1882. 


EARLY    HISTORY    OF    ANIMALS. 

BALFOUR,  F.  M.  "A  Treatise  on  Comparative  Embryology," 
Macmillan  &  Co.,  1880. 

BUTSCHLI.  "  Bemerkungen  zur  Gastraea  Theorie,"  Morph.  Jahr., 
1884. 

CONN,  H.  W.  "  Marine  Larvae  and  their  Relation  to  Adults," 
Biol.  Studies  of  J.  H.  U.  iii.,  1883.  "  A  Suggestion  from 
Modern  Embryology,"  Science,  1885. 

HAECKEL,  E.     "  Die  Gastraea  Theorie,"  Jena  Zeitschrift,   1874-5. 

HYATT,  A.  "Genesis  of  the  Arietidae,"  Smithsonian  Contributions 
to  Knowledge,  1889.  "  Larval  Theory  of  the  Origin  of  Cellular 
Tissue,"  Proc.  of  Bost.  Soc.  Nat.  Hist.,  1884. 

LANKESTER,  E.  R.  "  Notes  on  Embryology  and  Classification  of 
the  Animal  Kingdom,"  Quar.  Jour.  Mic.  Set.,  1877. 

McMuRRiCH.  "  The  Gastnea  Theory  and  its  Successors,"  Biologi- 
cal Lectures,  Ginn  &  Co.,  1890. 

MITSCHNIKOF.  "  Embryologische  Studien  an  Medusen,"  Wien, 
1886. 

SEDGWICK,  A.  "Origin  of  Metameric  Segmentation,"  Quar.  Jour. 
Mic.  Soc.,  1884. 

WILSON,  E.  B.  "Some  Problems  of  Annelid  Morphology,"  Bio- 
logical Lectures,  1890,  Ginn  &  Co. 

GEOLOGICAL    HISTORY. 

COPE,  E.  D.     "  Origin  of  the  Fittest,"  D.  Appleton  &  Co.,  1887. 
DANA,  R.     "  New  Text-book  of  Geology,"  Ivison,  Blakeman,  Tay- 
lor, &  Co.,  1883. 


REFERENCES.  191 

GEIKIE,  A.     "  Text-Book  of  Geology,"  Macmillan  &  Co.,  1885. 
LECONTE.     "  Elements  of  Geology,"  D.  Appleton  &  Co.,  1882. 
LYELL.     "  Principles  of  Geology,"  D.  Appleton  &  Co.,  1872. 
NICHOLSON  and  LYDECKER.     "A  Manual  of  Paleontology,"  Wm. 
Blackwood  &  Sons,  1889. 

PLANTS. 

DAWSON.     "  The  Geological  History  of  Plants,"  D.  Appleton  &  Co. 
NICHOLSON  and  LYDECKER.     "  Manual  of  Paleontology,"  vol.  ii. 
SACHS.     "  Text-book  of  Botany,"  Clarendon  Press,  1882. 

MISCELLANEOUS. 

CLAUS  and  SEDGWICK.  "  Elementary  Text-book  of  Zoology,"  Mac- 
millan &  Co.,  1884. 

CONN,  H.  W.  "Natural  Selection  and  Christianity,"  Methodist 
Review,  1891. 

DARWIN,  C.  "  Origin  of  Species,"  "Descent  of  Man,"  D.  Apple- 
ton  &  Co. 

FISKE,  J.  "  The  Destiny  of  Man,"  Houghton,  Mifflin,  &  Co.,  1885. 
"  Outlines  of  Cosmic  Philosophy,"  J.  R.  Osgood  &  Co.,  1875. 

HUXLEY,  TH.  "  Evidence  as  to  Man's  Place  in  Nature,"  New  York, 
1863. 

LUBBOCK,  J.      "  Pre-Historic  Times,"  London,  1869. 

MIVART,  ST.  GEO.  "  Genesis  of  Species,"  Macmillan  &  Co.,  1871. 
"  Lessons  from  Nature,"  D.  Appleton  &  Co.,  1876. 

ROMANES,  G.  "Mental  Evolution  in  Animals,"  1884  ;  "Mental 
Evolution  in  Man,"  1889,  D.  Appleton  &  Co. 

REED.     "  Evolution  versus  Involution,"  J.  Pott  &  Co.,  1885. 

SPENCER,  H.     "  Principles  of  Biology,"  D.  Appleton  £  Co. 

TYLOR.     "  Primitive  Culture,"  John  Murray,  London,  1871. 

WALLACE,  A.  "On  Natural  Selection,"  Macmillan  &  Co.,  1870. 
"  Darwinism,"  Macmillan  &  Co.,  1889. 


INDEX. 


Advance  of  type,  136 

Algae,  170 

Amphibia,  no 

Anatomy,  10 

Animals  and  plants,  divergence  of, 

67,  45 

Archean  age,  90 
Articulata,  107 
Assimilation,  23 

Beginnings  of  fossiliferous  history, 

93 

Birds,  history  of,  1 12 

Branching  arrangement  of  organ- 
isms, 66,  98 

Breaks  in  continuity,  48,  50 

Carbon  compounds,  42 
Carboniferous  age,  91,  115 
Cells,  59,  60 
Cenozoic,  92,  114 
Change  in  species,  116 
Chemistry  of  organic  compounds, 

19 

Chlorophyll,  17,  37 
Christianity,  law  of,  155 
Ccelentera,  history  of,  100 


Convergence  of  character,  152 
Cretaceous  age,  91 
Cycads,  172 

Death,  32 

Degeneration,  139 

Devonian  age,  91 

Difference  between  dead  and  living 

organism,  25 
Differentiation,  81 
Disappearance  of  types,  121 
Divergence,  of  character,   150  ;    of 

types,  77,  80  ;  prior  to  Silurian, 

95-97 

Diversity  of  modern  fauna,  138 
Division  of  cells,  65 
Dominant  types,  159 

Early  divergence  of  types,  80,  83 
Echinodermata,  history  of,  101 
Embryology,    6  ;    falsification    of, 

9  ;  imperfection  of,  7 
Endogens,  172 
Ethical  nature,  151 
Evolution,  47  ;  limits  of,  180 
Exogens,  172 
Expansion  of  types,  132 


193 


194 


Ferns,  171 

Fishes,  history  of,  no 

Fossils,  value  of,  3 

Gastraea,    70  ;    formation    of,    76 ; 

modification  of,  78  ;  significance 

of,  72 

Gastrula,  typical,  72 
Geological  ages,  88 
C Granules,  54 
Growth,  23 
Gymnosperms,  171 

Haeckel,  72 

Hard  parts,  development  of,  142 

Hydra,  figure  of,  74 

Influence  of  man  on  animals,  186 
Insects,  147  ;  history  of,  108 
Instinct,  146 
Intelligence,  147 
Intermediate  types,  122 

Jurassic  age,  91 

Law  of  continuity,  14,  47 
Life,  essential  characters  of,  17 
Limitation  of  differentiation,  82 
Limit  of  organic  development,  180 
Love,  among  animals,  152  ;  among 

men,  153 

Low  forms  of  life,  13 
Lycopods,  171 

Mammals,  expension  of,  134  ;  his- 
tory of,  112 

Man,  147  ;  capability  of  develop- 
ment, 187  ;  classification  of,  150 

Mechanical,  origin  of  life,  40 ; 
theory  of  life,  30 

Mental  factor,  145 


Mesozoic,  91,  114 
Metamorphosis,  5 
Modern  fauna,  paucity  of,  162  ; 

flora,  172 

Mollusca,  history  of,  104 
Molluscoidea,  history  of,  103 
Multicellular    animals,    origin    of, 

67 

New    fields    for    expansion,    132  ; 

lines  of  specialization,   183 
Nucleus,  61,  64 

Ocean  as  starting-point  of  life,  55 
i  Organism  as  a  machine,  26 
\  Origin  of  life,  33 

I  Paleozoic,  90,  114 
Permian  age,  91 
Persistent  species,  118 
Physical  basis  of  life,  51 
Physiological  chemistry,  12 
Plants  and  animals,    45  ;    history 

compared,  184 
Plants,  as  fossils,  170  ;  history  of, 

ID?,  173 

Properties  of  compounds,  31 
Protophyta,  62 
Protoplasm,  43,  51  ;  chemistry  of, 

52,  64  ;  structure  of,  52 
Protozoa,  62  ;  history  of,  99 

Quaternary  age,  92 

Rapidity  of  early  development,  81, 

178 

Redi,  35 

Reproduction,  24 
Reptiles,  history  of,  no 
Rhizocarps,  171 


INDEX. 


195 


Silurian,  age,  90  ;  life  of,  94 
Size,  increase  in,  144 
Skeleton,  development  of,  142 
Soft-bodied  animals,  141 
Specialization,  new  lines  of,  183 
Speculations  on  life,  value  of,  46 
Spontaneous  generation,  n,  35 

Terrestrial  life,  rarity  of,  160 
Tertiary,  92,  116 

Tree-like   arrangement    of    organ- 
isms, 164 


Triassic  age,  91 
Tyndall,  36 

j  Types,  of  fossil  and  living  animals, 
1 20  ;  history  of,  123 

Unicellular  life,  59,  64 

!  Vermes,  history  of,  107 
Vertebrata,  history  of,  no 
Vital  force,    29  ;  and  physical  en- 
ergy, 17 


UNIYEESITY  OF  CALIFORNIA  LIBEAEY 
BERKELEY 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
STAMPED  BELOW 

Books  not  returned  on  time  are  subject  to  a  fine  of 
50c  per  volume  after  the  third  day  overdue,  increasing 
to  $1.00  per  volume  after  the  sixth  day.  Books  not  in 
demand  may  be  renewed  if  application  is  made  before 
expiration  of  loan  period. 


OCT  30  1917 
JUL   8  1918 


MAY  13  19?? 


AUG  2  4  I960 
AG  30 '60  GO 


50m-7,'16 


~ 


UNIVERSITY  OF  CALIFORNIA  LIBRARY 


